The Clutch in an Automotive

So now that we have a basic idea of how gearboxes works there's a second item in transmissions that one needs to understand - the clutch. The clutch is what enables you to change gears, and keep the car idling at one point without switching off the engine. In the absence of the clutch the engine will have to be switched off every time we bring the vehicle to a stop. The clutch is needed because the engine is running all the time which means the crank is spinning all the time. We need some way to disconnect this constantly-spinning crank from the gearbox, both to allow us to stand still as well as to allow us to change gears. This is where the clutch comes in.

The clutch is composed of three basic elements;                                                                             

1.The flywheel,

2.The pressure plate and

3.The clutch plate(s).

 

 

 

In an automobile clutch, the flywheel is connected to the engine, and the clutch plate is connected to the transmission. In normal running they are in contact with each other and thereby transmit the power. The clutch cover is bolted to the flywheel so it turns with the flywheel.  The Diaphragm spring is attached to the clutch cover housing using a pivot mechanism.

The ends of the diaphragm springs are hooked under the lip of the pressure plate. So as the engine turns, the flywheel, clutch cover, diaphragm springs and pressure plate are all spinning together. The clutch pedal is connected either mechanically or hydraulically to a fork mechanism which loops around the throw-out bearing.

Case 1) When the clutch pedal is pressed.
The clutch pedal is connected either mechanically or hydraulically to a fork mechanism which loops around the throw-out bearing. When the clutch pedal is pressed, a cable or hydraulic piston pushes on the release fork, which presses the throw-out bearing against the middle of the diaphragm spring. As the middle of the diaphragm spring is pushed in, a series of pins near the outside of the spring cause the spring to pull the pressure plate away from the clutch disc. This release of pressure allows the clutch plates to disengage from the flywheel. Now the clutch plates are disengaged from the flywheel and this permits the vehicle to be stationary while the engine is running. The flywheel keeps spinning on the end of the engine crank but it no longer drives the gearbox because the clutch plates aren't pressed up against it

.

Case 2) When the clutch pedal is released.

When the clutch pedal is released the exact opposite happens. As you start to release the clutch pedal, pressure is released on the throw-out bearing and the diaphragm springs begin to push the pressure plate back against the back of the clutch plates, in turn pushing them against the flywheel again. Springs inside the clutch plate absorb the initial shock of the clutch touching the flywheel and as you take your foot off the clutch pedal completely, the clutch is firmly pressed against it. The friction material on the clutch plate is what grips the back of the flywheel and causes the input shaft of the gearbox to spin at the same speed.
The amount of force the clutch can hold depends on the friction between the clutch plate and the flywheel, and how much force the spring puts on the pressure plate.

Working of a Gearbox

How does a gearbox work?

In the picture below we can see the internals of a normal gearbox. It consists of helical gears meshing with each other. There are two shafts in the gearbox.

The lower shaft in this image is called the layshaft - it's the one connected to the clutch - the one driven directly by the engine. The output shaft is the upper shaft in this image. By looking at the image we can determine that it is a 5 speed gearbox.

 

This is understood by looking at the output shaft .It has 5 helical gears and 3 sets of selector forks, that tells this is a 5-speed box. When the clutch is engaged, the layshaft is always turning. All the helical gears on the layshaft are permanently attached to it so they all turn at the same rate. They mesh with a series of gears on the output shaft that are mounted on slip rings so they actually spin around the output shaft without turning it. This means that they can spin independently without turning either the output shaft or the other gears on the output shaft.

As long as any gear is not engaged the gears on the output shaft it keeps spinning without turning the output shaft. When any gear is selected the following actions are done, the selector forks which are visible between the gears are slipped around a series of collars with teeth on the inside. Those are the dog gears and the teeth are the dog teeth. The dog gears are mounted to the output shaft on a splined section which allows them to slide back and forth. When a gear is engaged, a series of mechanical pushrod connections move the various selector forks, sliding the dog gears back and forth, thereby engaging the gears. Once the dog gears are engaged then the gear on the output shaft will start to spin and as the dog gear is locking the output shaft with the gear the output shaft also spins with it. When the next gear is selected the dog gear moves to the next gear thereby causing the output shaft to spin at a different speed.

 

 

 

 

 

 

 

What about reverse?

So far we have seen how the gearbox works. Since all the forward gears are directly meshed with the output shaft they will all turn in the same direction. However in the case of the reverse gear there is a requirement to reverse the direction of rotation.

This requirement is met by introducing an idler gear in between the output shaft and the layshaft which ensures that the output shaft and the layshaft spin in the same direction. Typically, there will be three gears that mesh together at one point in the gearbox instead of the customary two. There will be a gear each on the layshaft and output shaft as usual, but there will be a small gear in between them called the idler gear. The inclusion of this extra mini gear causes the last helical gear on the output shaft to spin in the opposite direction to all the others. The principle of engaging reverse is the same as for any other gear - a dog gear is slid into place with a selector fork. Because the reverse gear is spinning in the opposite direction, when the clutch is releases, the gearbox output shaft spins the other way - in reverse.  

 

Introduction to Gear/Gearing/Gear box/Gear Ratio

Introduction to Gear/Gearing/Gear box/Gear Ratio

Before going to the system we will first understand some basic definitions about the system components.

Gear is a toothed wheel that engages another toothed mechanism in order to change the speed or direction of transmitted motion.

Gearing is wheelwork consisting of a connected set of rotating gears by which force is transmitted or motion or torque is changed.

Gearbox is an automotive assembly of gears and associated parts by which power is transmitted from the engine to a driving axle. The term gearbox is sometimes also refers as transmission.

Gear Ratio is the numeric indication of the relationship of number of teeth on two gears. In other terms it can also define as, the ratio of the speeds of rotation of the initial and final gears in a gear train

Introduction to Transmission system

When two or more gears working in tandem, it is called a transmission. In automotive system, transmission generally refers to the whole drive train, including gearbox, clutch, prop shaft (for rear-wheel drive), differential and final drive shafts. As the name suggests it is a system to transfer the motive power from the engine to the wheels. The most common use is in the motor vehicles, where the transmission adapts the output of the internal combustion engine to the drive wheels.

Need of gears and gearboxes

A gearbox provides speed and torque conversions from a rotating power source to another device using gear ratios. Such engines need to operate at a relatively high rotational speed, which is inappropriate for starting, stopping, and slower travel.

This is where the transmission comes in; the transmission reduces the higher engine speed to the slower wheel speed, increasing torque in the process.

 Most modern gearboxes are used to increase torque while reducing the speed of a prime mover output shaft (e.g. a motor crankshaft). This means that the output shaft of a gearbox will rotate at slower rate than the input shaft, and this reduction in speed will produce a mechanical advantage, causing an increase in torque. A gearbox can be setup to do the opposite and provide an increase in shaft speed with a reduction of torque. Some of the simplest gearboxes merely change the physical direction in which power is transmitted.

Many typical automobile transmissions include the ability to select one of several different gear ratios. In this case, most of the gear ratios (often simply called "gears") are used to slow down the output speed of the engine and increase torque. However, the highest gears may be "overdrive" types that increase the output speed.

Working of gears and gear systems

The following is a simplified working of gears and gear systems.

 The number of teeth cut into the edge of a gear determines its scalar relative to other gears in a mechanical system. For example, if you mesh together a 20-tooth gear and a 10-tooth gear, then drive the 20-tooth gear for one rotation; it will cause the 10-tooth gear to turn twice.

Gear ratios are calculated by dividing the number of teeth on the output gear by the number of teeth on the input gear. So the gear ratio here is output/input, 10/20 = 1/2 = 1:2. Gear ratios are often simplified to represent the number of times the output gear has to turn once. In this example, 1:2 is 0.5:1 - "point five to one". Meaning the input gear has to spin half a revolution to drive the output gear once. This is known as gearing up.

Gearing down is exactly the same only the input gear is now the one with the least number of teeth. In this case, driving the 10-tooth gear as the input gear gives us output/input of 20/10 = 2/1 = 2:1 - "two to one". Meaning the input gear has to spin twice to drive the output gear once. By meshing many gears together of different sizes, you can create a mechanical system to gear up or gear down the number of rotations very quickly. As a final example, imagine an input gear with 10 teeth, a secondary gear with 20 teeth and a final gear with 30 teeth. From the input gear to the secondary gear, the ratio is 20/10 = 2:1. From the second gear to the final gear, the ratio is 30/20 = 1.5:1. The total gear ratio for this system is (2 * 1.5):1, or 3:1. i.e. to turn the output gear once, the input gear has to turn three times.

 

 

 

 

 

 

Collections of helical gears in a gearbox are what give the gearing down of the speed of the engine crank to the final speed of the output shaft from the gearbox. The table below shows some example gear ratios for a 5-speed manual gearbox.

 

 

Gear	Ratio	RPM of gearbox output shaft when the engine is at 3000rpm
1st	3.166:1	947
2nd	1.882:1	1594
3rd	1.296:1	2314
4th	0.972:1	3086
5th	0.738:1	4065

Here you will see that the 4^th and 5^th gear are overdriven, i.e. the gearbox output shaft speed is higher than the engine speed.  From the table we see how we can effectively obtain 5 different output speeds even when the input speed remains constant.

In the next part of the series we shall look at the working of a gearbox and other components of a transmission.

 

 

Disclaimer

The article doesn't include any confidential information. Nor does it include any references from the sources which doesn't allow taking excerpts from articles. The images have been taken from internet not necessarily from direct source. The article is only for reference and educational purpose only.

 

Variable Valve Timing (VVT)

Variable Valve Timing

In internal combustion engines, variable valve timing, often abbreviated to VVT, is a generic term for an automobile piston engine technology. VVT allows the lift, duration or timing (in various combinations) of the intake and/or exhaust valves to be changed while the engine is in operation. Variable Valve Timing & Lift improve engine efficiency by optimizing the flow of fuel & air into the engine for various engine speeds. Two-stroke engines use a power valve system to get similar results to VVT.

Working of a Non-VVT Engine

Piston engines normally use poppet valves for intake and exhaust. These are driven by cams on a camshaft. The cams open the valves (lift) for a certain amount of time (duration) during each intake and exhaust cycle. The timing of the valve opening and closing is also important. The camshaft is driven by the crankshaft through timing belts, gears or chains.

 

The profile, or position and shape of the cam lobes on the shaft, is optimized for a certain engine revolutions per minute (RPM), and this tradeoff normally limits low-end torque, or high-end power. In a non-VVT engine, as the RPM increases, the time the valve stays open also becomes less (so in a way the open time is variable with RPM but opens and closes at a faster rate. At high engine speeds, an engine requires large amounts of air. However, the intake valves may close before all the air has been given a chance to flow in, reducing performance. On the other hand, if the cam keeps the valves open for longer periods of time, as with a racing cam, problems start to occur at the lower engine speeds. This will cause unburnt fuel to exit the engine since the valves are still open. This leads to lower engine performance and increased emissions.

 

Working of VVT Engine

In VVT, the time the intake valves stay open is controlled by other means in addition to that based on the engine's RPM. VVT allows the cam timing to change, which results in greater efficiency and power, over a wider range of engine RPMs. With VVT a valve may be made to stay open longer with increasing RPM. The overall effect is that the engine can breathe better and along with fuel injection can give better performance/mileage.

 

VVT is achieved by having more than one cam profile per valve and selecting a cam based on the RPM ranges (or load conditions), such that at the lowest range (or load condition) the cam profile chosen will be the one that ensure minimum air required to maintain the RPM. The ECU will still inject fuel based on the amount of air sucked but this time the amount of fuel will be less as compared to the non-VVT engine. Hence we can see that the engine will burn lesser fuel and hence give better mileage.

 

Need of VVT

Pressure to meet environmental goals and fuel efficiency standards is forcing car manufacturers to turn to VVT as a solution. Most simple VVT systems advance or retard the timing of the intake or exhaust valves. Others (like Honda's VTEC) switch between two sets of cam lobes at a certain engine RPM. Furthermore Honda's i-VTEC can alter intake valve timing continuously.

 

Alternative to VVT

It should not be taken that a non-VVT engine is bad. These too have techniques to ensure efficiency. Consider the low RPM example I gave, the ECU can detect the load conditions/RPM and ensure a lean or ultra-lean burn (i.e., inject lesser fuel than that required by the ideal air-fuel ratio). This will also improve efficiency and is a technique commonly used.

 

Convert car from non VVT to VVT:-

It would be impractical to try converting a non-VVT engine to VVT - at least where reasonable budget it concerned, not to mention the time/effort required. Better would be to replace the engine itself (though this too won't be cheap)

 

Systems with VVT Implementation

As I told earlier VVT is a generic term for an automobile technology. This allows the lift, duration or timing of the intake and/or exhaust valves to be changed while the engine is in operation

 

There are various systems developed by Automobiles manufacturers to implement VVT in there system such as VVT-i, VVTL-i by Toyota, VTEC by Honda, CVVT by Volvo, VANOS by BMW etc.

Some can be only use to control one valve (single VVT) while other are to control both valve (dual VVT).

I will explain about few of them.

VVT-i  (Variable Valve Timing with Intelligence)

VVT-i is an implementation of VVT technology in an intelligent way using microprocessors to control the VVT functionality using some actuators.  VVT-i was developed by Toyota and came into implementation from 1996 which bring variations in the time of the intake and exhaust valves. This is an automobile variable valve timing technology which is quite similar to BMW's VANOS and was aimed at replacing the Toyota's VVT technology introduced in 1991 for 4A-GE engines.

The technology is responsible for variations in the timing of the intake valves by making adjustments in the mechanisms between the camshaft drive(belt, chain etc) and the intake camshaft. The medium of these adjustments is the Engine Oil pressure which is applied to the actuator for adjusting the camshaft position. Adjustments made in the overlap time in between the opening of intake valves and closing of exhaust valves is responsible for higher efficiency of the engine. Several variants of this system have been introduced since the introduction of VVT-i including VVTL-i, Dual VVT-i, VVT-iE and Valvematic.

VVTLi (Variable Valve Timing and Lift Intelligent System)

VVTL-i is a system whose principle of functionality has been derived from VVT-i but it is different in a way that it alters the valve timing as well as the valve lift(duration).

Dual VVT-i

Dual VVT-i as the name suggests would carry out the same function but on two valves at the same time. Dual VVT-i was introduced in 1998 in 3S-GE engines which not only alters the timing of the intake valves but also of the exhaust valves camshafts.

VVT-iE
VVT-iE stands for Variable valve timing – intelligent by electric motor. Being a variant of the Dual VVT-i technology this technology adjusts and maintains the intake camshaft timing with the help of electrically operated actuator.

VTEC
Honda also developed a technology of its own called the VTEC(Variable Valve Timing and Lift Electronic Control) which not altered the camshaft timings but also made an engine to have multiple camshafts rendering it with better performances.

Application of VVT

 

VVT technology is utilized in all types of internal combustion engines. It can be implemented for Gasoline, Diesel & CNG engines etc.

 

VVT Vehicles in India

 There are many cars where VVT is implemented worldwide. I would like to mention few where VVT technology is implemented. Although it may be be implemented in All cars of below series but some particular models only.

 

Vehicles in India

Maruti Suzuki SX4

Toyota Corolla

TATA Safari

 

Future vehicle with VVT in India

Toyota Fortuner

Maruti Cervo*

Tata Nano**

 

* Maruti cervo is expected to be competitor of Tata Nano, Its already launched in Japan, but no official news if it will be launched in India

* I heard one of high end model of Tata Nano is planned to have VVT, but not sure when)

Fuel Energy consumption in a typical Gasoline vehicle

Fuel Energy consumption in a typical Gasoline vehicle

As we discussed in last article, in a typical Petrol (Gasoline) vehicle only ~15% of energy is utilized to drive the vehicle. As to understand where we can improve the energy efficiency it is important to know where we are having maximum losses. Once again we will go through where is the fuel energy consumed in a typical Petrol (Gasoline) vehicles.

 (Data source: U.S. Dept. of Energy)

Diesel engines generally achieve greater fuel efficiency than petrol (gasoline) engines because of high energy efficiency of Diesel engines and latest advancement in diesel technology.

 *Currently I am not having exact data for diesel engine from a reliable source. once available I will share it later.

Latest Technologies in the Automotive field 

The majority of the chemical energy of is lost by the engine and driveline inefficiencies. Current focus is on improving powertrain efficiency. In this article we will discuss what all are the upcoming technologies and how much fuel economy improvement it offers.

Most of the new technologies come in below two categories

-          Engine Technologies (which improves engine efficiencies)

-          Transmission Technologies (which improves transmission efficiencies)

Apart from these two categories there can be some more technologies such as Improved aerodynamic design, latest tires technology (to reduce rolling resistance), better alternator design to reduce loss in accessories etc. by which we can improve the fuel economy of the vehicle. 

Engine Technologies

Variable Valve Timing & Lift (5%)

Variable Valve Timing & Lift improve engine efficiency by optimizing the flow of fuel & air into the engine for various engine speeds. It is a generic term for an automobile piston engine technology. VVT allows the lift, duration or timing (in various combinations) of the intake and/or exhaust valves to be changed while the engine is in operation.

Cylinder Deactivation (7.5%)

Cylinder deactivation is used to reduce the fuel consumption and emissions of an internal combustion engine during light load operation. In typical light load driving the driver uses only around 30 percent of an engine's maximum power. During these conditions the full displacement of engine is not required, the displacement can be reduced the by deactivating some cylinders. Cylinder deactivation will reduce pumping losses and result in better fuel efficiency. It is achieved by keeping the intake and exhaust valves closed for a particular cylinder.

Turbochargers & Superchargers (7.5%)

Turbochargers and superchargers are fans that force compressed air into an engine's cylinders. A turbocharger fan is powered by exhaust from the engine, while a supercharger fan is powered by the engine itself.  Both technologies allow more compressed air and fuel to be injected into the cylinders, generating extra power from each explosion. This allows manufacturers to use smaller engines without sacrificing performance or to increase performance without lowering fuel economy.

Integrated Starter/Generator (ISG) Systems (8%)

Integrated Starter/Generator (ISG) Systems also known as Start-Stop system automatically turn the engine off when the vehicle comes to a stop and restart it instantaneously when the accelerator is pressed so that fuel isn't wasted for idling. In addition, regenerative braking is often used to convert mechanical energy lost in braking into electricity, which is stored in a battery and used to power the automatic starter.

Direct Fuel Injection (11-13%)

In conventional multi-port fuel injection systems, fuel is injected into the port and mixed with air before the air-fuel mixture is pumped into the cylinder. In direct injection systems, fuel is injected directly into the cylinder so that the timing and shape of the fuel mist can be precisely controlled. This allows higher compression ratios and more efficient fuel intake, which deliver higher performance with lower fuel consumption.

Variable Compression (9%)

A variable compression ratio (VCR) engine is able to operate at different compression ratios, depending on a particular vehicle's needs. Thus, a VCR engine is optimized for the full range of driving conditions, such as acceleration, speed, and load.

HCCI (15%)

Presently, the overriding ICE development goals are to make the diesel engine as clean as the gasoline engine, while making the gasoline engine as efficient as the diesel engine. HCCI (Homogeneous Charge Compression Ignition) engines combine elements of both.

Switching to Diesel (30%)

Diesel engines have better fuel economy per volumetric measure than typical gasoline engines by virtue of diesel's greater energy density. The fuel savings of diesel versus gasoline are typically in the 20-30% range.

 

Hybridization (5-30%)

A hybrid vehicle is a vehicle with at least two different energy converters and two different energy storage systems (on-board the vehicle) for the purpose of vehicle propulsion. Electric power is the mostly used as second source of population. So term Hybrid vehicle is mostly used for hybrid electric vehicle. Hybridization can play a major part in improving vehicles fuel efficiency.

Transmission Technologies

Continuously Variable Transmissions (CVTs) (6%)

Most conventional transmission systems control the ratio between engine speed and wheel speed using a fixed number of metal gears. Rather than using gears, the CVTs use a pair of variable-diameter pulleys connected by a belt or chain that can produce an infinite number of engine/wheel speed ratios.

Automated Manual Transmissions (AMTs) (6%)

Automated Manual Transmissions (AMTs) combine the efficiency of manual transmissions with the convenience of automatics (gears shift automatically).

Manual transmissions are lighter than conventional automatic transmissions and suffer fewer energy losses. However, most drivers prefer the convenience of an automatic. AMT operates similarly to a manual transmission except that it does not require clutch actuation or shifting by the driver. Automatic shifting is controlled electronically (shift-by-wire) and performed by a hydraulic system or electric motor. In addition, technologies can be employed to make the shifting process smoother than conventional manual transmissions.

Other Technologies

Vehicle Downsizing & Improving Aerodynamic

Meeting future emission standards will also impact vehicle design. The impact will likely be smaller, lighter vehicles that require less energy to operate. Also the aerodynamics will be improved to get better fuel efficiency as it can reduce air drag to significant limits.

We will cover each and every technology in details in future articles.

**Few of the above technologies are only applicable to one kind of engine Gasoline or Diesel while others are applicable to both. We will discuss about that later

Energy Losses in a typical Gasoline Vehicle

Energy Losses in a typical Gasoline Vehicle

Only about 15% of the energy from the fuel you put in your tank is used to move your car down the road or run useful accessories, such as air conditioning. The rest of the energy is lost to engine and driveline inefficiencies and idling. Therefore, the potential to improve fuel efficiency with advanced technologies is enormous.

Source: U.S. Dept. of Energy

Engine Losses - 62.4 percent
In gasoline-powered vehicles, over 62 percent of the fuel's energy is lost in the internal combustion engine (ICE). ICE engines are very inefficient at converting the fuel's chemical energy to mechanical energy, losing energy to engine friction, pumping air into and out of the engine, and wasted heat.

Advanced engine technologies such as variable valve timing and lift, turbo charging, direct fuel injection, and cylinder deactivation can be used to reduce these losses.

In addition, diesels are about 30-35 percent more efficient than gasoline engines, and new advances in diesel technologies and fuels are making these vehicles more attractive.

Idling Losses - 17.2 percent
In urban driving, significant energy is lost to idling at stop lights or in traffic. Technologies such as Start Stop systems help reduce these losses by automatically turning the engine off when the vehicle comes to a stop and restarting it instantaneously when the accelerator is pressed.

Accessories - 2.2 percent
Air conditioning, power steering, windshield wipers, and other accessories use energy generated from the engine. Fuel economy improvements of up to 1 percent may be achievable with more efficient alternator systems and power steering pumps.

Driveline Losses - 5.6 percent
Energy is lost in the transmission and other parts of the driveline. Technologies, such as automated manual transmission and continuously variable transmission, are being developed to reduce these losses.

Aerodynamic Drag - 2.6 percent
A vehicle must expend energy to move air out of the way as it goes down the road,less energy at lower speeds and progressively more as speed increases. Drag is directly related to the vehicle's shape. Smoother vehicle shapes have already reduced drag significantly, but further reductions of 20-30 percent are possible.

Rolling Resistance - 4.2 percent
Rolling resistance is a measure of the force necessary to move the tire forward and is directly proportional to the weight of the load supported by the tire. A variety of new technologies can be used to reduce rolling resistance, including improved tire tread and shoulder designs and materials used in the tire belt and traction surfaces.

For passenger cars, a 5-7 percent reduction in rolling resistance increases fuel efficiency by 1 percent. However, these improvements must be balanced against traction, durabillity, and noise.

Overcoming Inertia; Braking Losses - 5.8 percent
To move forward, a vehicle's drivetrain must provide enough energy to overcome the vehicle's inertia, which is directly related to its weight. The less a vehicle weighs, the less energy it takes to move it. Weight can be reduced by using lightweight materials and lighter-weight technologies (e.g., automated manual transmissions weigh less than conventional automatics).

In addition, any time you use your brakes, energy initially used to overcome inertia is lost.

Type of Losses

We can divide these vehicular losses into three basic types as follows:

Friction
Wind resistance, rolling resistance, mechanical friction (two parts moving against each other), and hydraulic friction (force required to push or pump a liquid)

Mechanical Load
The combination of losses incurred by the engine due to powering accessories such as air conditioning, electrical, and power steering as well as vehicle weight and payload

Heat & Noise
loss of energy in the form of heat loss or vibrational noise

Tips and Techniques for Reducing Friction Losses

Tip #1 Keep your tires inflated properly
Make sure to check your tire pressure any time there is a change in temperature outside and keep them filled up near the highest pressure listed on the tire. It's good practice to check them every week. This may be one of the most important ways for you to keep your fuel mileage high. Remember, adjusting your tire pressure for your specific vehicle load is important for optimum fuel mileage. Both safety and efficiency are clearly at risk if this is not proper.

Check for abnormal wear, typically along the center line of the tire if over-inflated and along the edges if under-inflated. Uneven wear or cupping of the tread along one or both edges is an indication of an alignment problem or an issue with your shocks or struts.

Tip #2 Don't drive with your windows down
In general experts suggest driving without Air Conditioning unless you really need it. There is a trade-off here. If it is too warm to keep the A/C off and keep your windows up, you are better off running your A/C than you are driving with at highway speeds with your windows down because of the excessive drag created by the open windows.

Tip #3 Apply synthetic bearing grease to your wheel bearings
Good quality synthetic bearing grease will greatly reduce the friction around your wheel bearings and will help decrease overall rolling resistance.

Tip #4 Use synthetic oil in your transmission, differential and transfer case
Good quality synthetic ATF or appropriate gear oil will dramatically reduce the friction and operating temperature inside your automatic or manual transmission, differential(s) or transfer case. It could also last up to 50,000 miles, or 3 to 5 times longer than conventional petroleum based oils.

Tip #5 Make sure your wheels are aligned to reduce rolling resistance
Maintaining your vehicle's alignment is paramount to the operational efficiency and safety of your vehicle. It should be in alignment and as level and straight as the level road you are traveling down. If you have a "cantered" steering wheel while driving straight down the road, you are probably in need of an alignment

Many times you might have bumped a curb or hit a pretty significant bump. Even one small uneventful bump from a curb can significantly mis-align your car.

It can also cause premature wearing of your tires. This can be a double whammy in that you pay more out of your pocket for each inch you travel because of poor mileage caused by an out of alignment vehicle AND having to buy new tires more often. Sounds a bit simplistic but this is a commonly overlooked mileage eater.

Tip #6 Maintain your brakes and make sure your emergency brake is disengaged
There are two parts to the brake equation - your standard brakes and your emergency brakes. Both are absolute critical systems and the highest priority should be given to their maintenance and effectiveness.

The worn and improperly maintained brakes can cause excess drag and may become a driving hazard. The brake pad contact points can fall out of alignment, ever so slightly, and create unwanted friction between the pads and the rotors. In most cases, you will hear a slight chatter or chirp telling you there is a problem. You may also feel this as slight vibration or chatter with your foot as you apply pressure on the brakes. Additionally, today's newer cars, with 4- wheel disc brakes, have a second-set of brake shoes (emergency brakes), used to keep the car from rolling when parked and to stop the car in the event the primary system has failed.

All too many times drivers leave the parking brake partially engaged and do not figure this out until they see smoke. If the emergency brakes get so hot they begin to fuse the shoe to the friction material it is in contact with, you may feel a distinct chatter or vibration in the car.

Tip #7 Make your vehicle more aerodynamic
Anything attached to the outside of your vehicle will add extra wind resistance. A car-top carrier, a bike rack on the back, even your luggage rack will add wind resistance. If your luggage rack is removable, take it off when you don't need it.

If you can add an air dam, spoilers, or fairings around the bottom of the vehicle to minimize the amount of air that can pass underneath, the vehicle will be more aerodynamic and the engine won't need to work so hard to move it. Obviously, if you truly go off-road in your SUV or truck then this may not be the option for you.