How to reduce your vehicle's fuel consumption

From IWillTry

Originally posted by Trebuchet03 at http://www.instructables.com on Aug 19, 2007 - republished here with permission on Sept 13, 2007.
Todo: add more (see Todo for sources)

Drive less

The best way to save on fuel is to not use it at all - walk, ride a bike, use public transportation, just stay home, etc. However, there is a rather large subset of the population that live in a suburban prison. The following is a set of techniques used by people that call themselves hypermilers to achieve high fuel economy/efficiency (FE) numbers, save the planet and save their wallets.

Tires

Inflate your tires to, at least, the maximum inflation pressure to reduce rolling resistance. Should you wish to go beyond, that is up to you. Personally, I inflate my tires to around 55psi (they are rated for 44). Please do not attempt on old or visibly damaged tires. My tires are new and in excellent condition.

Modern tires are radially belted - that is, they won't balloon like tires of yore. Keep in mind what you're doing though. You are reducing the contact patch of your tires in order to decrease static friction. This will reduce your overall braking ability. So riding someone's bumper at 80mph is just that much more dangerous. Again, these steps only work when combined - you can't pick and choose.

When it comes time to buy new tires, look for low rolling resistance (LRR) tires. While they may not have the same 60,000 mile (or even 40,000) guarantee, they will save you in the fuel department. The Toyota Prius comes equipped with LRR tires.

Weight

Take everything out of your car.... Then put back only what you need. For those more daring, remove your emergency jack and spare tire and be sure to carry a cell phone in case of emergency. AAA is a good thing to invest in - even if you don't plan on following these mileage techniques. If you won't be carrying passengers, remove your rear seat and passenger seat as this will lighten your load by a couple hundred pounds.

Maintenance

Keep your engine in Tune and use the lightest weight of oil as possible. In addition to friction, you also have to combat viscous forces. The thicker the oil, the more resistance your engine has to overcome.

O2 sensors are very critical pieces of equipment - especially the 1st one (primary).

If you have a check engine light (CEL) and your car is a 1996+ model (for US vehicles) - get your car scanned. Certain auto parts places will scan codes for free (such as Autozone). Fixing parts yourself can save a lot of money - that is, only if you feel comfortable doing it.

Reduce "non-essential" loads

You can't get something from nothing. The greater the electrical load you put on your alternator, the harder your engine has to work to turn it, and the more fuel you will consume. Avoid using the following:

Defrosters
Loud Sound Systems
Headlights - Disable daytime running lights if possible and legal where you live. Check a schematic of your electrical system for details.
Air Conditioning - This is not electrical, but still an additional load on the engine.
Fan - The choice between operating your fan, or opening a window is not clear cut. At high speeds, opening your window will significantly increase drag, which will consume more fuel than turning on your fan with the windows closed. At low speeds, opening your window is likely better.

Instrumentation

Install a trip computer that displays instantaneous fuel consumption. Then, turn driving into a game - always try to beat your highest score. The EPA (I believe) estimates something like a 1.5-3% fuel savings for on board fuel consumption instrumentation. A popular model is the Scangauge.

Idle free Stopping

It's been said that the amount of fuel required to start a modern vehicle is less than that required to idle for 10 seconds. See How many seconds of idling is equivalent to starting your engine? for experimental results. Furthermore, most of the extra fuel consumed by starting is the due to a faster idle speed setting that kicks in for the first 20-30 seconds. If you shift into gear and start moving within a couple seconds after starting, then the faster idle setting doesn't cost you any additional fuel since you are extracting useful work from it. If you start moving right away, then starting your engine consumes less fuel than 1 second of idling. Therefore, you can save fuel by turning your engine off whenever you don't need it. You do not even need to consider how long it will be before you have to restart.

Engine off Coasting

This is best attempted only on smaller vehicles without power steering and on which adequate braking is possible without power assist. Simply put, turn your engine off whenever you don't need it, even if you are still moving. For example, turn your engine off when you are coasting down hill, or coasting to a stop light, or decelerating for any other reason.

Pulse and Glide

Assume you want to average 35mph. Start at 40mph and allow your vehicle to decelerate, in Neutral, to 30mph -- this is called the glide. Then, accelerate back up to 40mph as efficiently as possible (see the acceleration section later) -- this is called the pulse. For maximum fuel efficiency, use engine off coasting during the glide. This technique will likely increase your fuel efficiency more than any other method in this article. It has been proven many times over to be an effective way to increase mileage. However, it is not conducive to driving in traffic. Use your best judgment and try not to annoy other drivers.

Hills

Avoid accelerating up hill. Try to coast up hill or, at the very least, maintain constant throttle (not speed). Then, while going back down the hill, accelerate back to your cruising speed. Consider shifting into neutral down hills if the slope is enough to maintain speed. If you are comfortable doing so, turn your engine off down hills.

Traffic lights, traffic, stop signs

The instant you see a red light from a brake light or the yellow/red light from a traffic signal - take your foot off the gas and coast. It's no use speeding up into a stop. Turn your engine off if you are comfortable doing so. For more advanced drivers, the most efficient action is actually to brake suddenly, dropping to a speed that you can maintain until the obstacle is gone (in the example of a red light, you want to drop immediately to the exact speed that will bring you to the intersection just as the light turns green). However, this may be perceived as erratic behavior by other drivers, so use your best judgement.

Acceleration

The best way to accelerate is by keeping your engine at peak torque. This means that accelerating gently (as others may have recommended) is inefficient. Accelerating rapidly (low gear, high RPMs) is also inefficient. A more efficient way to accelerate is in high gear, at low RPMS and 50-75% throttle.

To look at the reason for this, let's define a few terms:

E_in        = Energy produced by combustion on each engine revolution (increases with throttle position)
E_f         = Energy lost to engine friction on each engine revolution (approximately constant)
E_out       = Energy output to drive train on each engine revolution = E_in - E_f
Efficiency = E_out / E_in = 1 - E_f/E_in

There are a few results from these simple formulas.

High RPMs are bad. Total energy lost to engine friction over a given distance is proportional to the number of engine revolutions taken to travel that distance. To reduce losses due to engine friction, you want to be in the highest gear possible (without overtaxing the engine), so that your engine makes a few revolutions as possible.
Light throttle is bad. E_in is only slightly greater than E_f, and thus the efficiency is low. Only a small percentage of the energy produced per revolution is transfered to the drive train. Most of the energy is consumed by engine friction. Effectively this is only a little better than idling.
Heavy throttle at low RPM (accelerating in high gear) is good. E_in is much greater than E_f, and thus efficiency is high. A larger percentage of the energy produced is transfered to the drive train.

Here are some general rules of thumb for efficient acceleration:

Shift into the highest gear at which you can accelerate at 50% throttle without overtaxing the engine.
Keep your gas pedal at the point of maximum acceleration. If going from 25% throttle to 50% throttle doesn't increase your acceleration, then stay at 25%. This point will change as your RPMs rise.
Keep your RPMs under 2000. When your RPMs reach 2000, shift up to the next gear.
When you reach your target speed, shift into your highest gear and adjust throttle position to maintain constant speed. If it feels like you have no power in such a high gear, that's exactly what you want. It means that the engine is producing the maximum useful energy per revolution. If you need to accelerate further from this speed, you can drop down a gear.

Aerodynamics

Aerodynamics are important to a car. It's hard to make a car better aerodynamically - but it's very easy to make it worse. Some techniques hypermilers use are:

Blocking your grill. This has proven to be effective. Your radiator (and grill opening) were designed for the worst case cooling scenario. Monitor your engine temperature and adjust your grill opening accordingly.
Folding your mirrors. If you can, fold your outside mirrors in. Just be sure to stay in compliance with local laws. Some hypermilers remove their outside side-view mirrors completely relying only on their rear-view mirror.
Add a belly pan. A significant amount of drag results from the open areas under your car. Covering them with corroplast or some other material can reduce your drag coefficient (Cd).
Add wheel skirts. Covering your wheel wells to keep your car's body one contiguous object can help considerably.
Add an air dam. Basically, block off the area between your front bumper and the ground. If you car doesn't have one already, this can help.
Always drive with your windows up.
Slow down. The force of air resistance increases as the square of your velocity (holding everything else equal).

Pretend you have no brakes

If you didn't have brakes, you'd only go as fast as you needed to ensure you could stop at your next destination. That is, don't apply so much power that you'll only be throwing away your energy VIA braking. Coasting is your friend.

Join a Community

Find an online community to talk with and collaborate. There are plenty of experienced and knowledgeable people out there. Personally, I'm fond of the people at http://www.gassavers.org. You'll find plenty of experiments that, quite commendably, follow a scientific method.

Disclaimer

Some of the techniques described above are arguably dangerous, both to the vehicle occupants and to other drivers on the road. Once a driver becomes familiar with a vehicle's behavior in response to these techniques, the danger is minimal, but there is a learning curve. In particular, many readers will likely argue the dangers of turning your engine off while your vehicle is moving because the power brakes and power steering (if equipped) will cease to function. In larger vehicles this may be a serious issue, and this technique should not be used without installing proper equipment to maintain power assist. However, smaller vehicles often do not have power steering or power brakes, and even if equipped, may not be hindered significantly if the power assist is disabled. Also, one would be equally justified in arguing that it is dangerous for any driver not to be familiar with how their vehicle handles when the engine stops, for whatever reason. Use your own judgment, and be careful.

External Links

www.gassavers.org - an online community of fuel economy enthusiasts and a great resource
www.metrompg.com - a source of info about DIY efficiency mods, fuel-efficient driving, and fuel economy in general
www.crxmpg.com - a fuel economy resource for everyone (as taught by the Honda CRX)