Kgwedi's Basic Fuel Economy Tips For Newbies
This is a quick and simple introduction to the art of getting good fuel economy from a motor vehicle.
It is important to understand that the theory assumes ideal conditions, which we seldom have. So when I say such and such is true, it does not mean it will always apply in day to day driving.
When it comes to improvements you do to your car, or your driving procedure, there will usually be very small improvements to your fuel economy. Nothing will help a lot, but every improvement will help a little bit. This means that good fuel economy is a result of a persistent attitude, rather than of one modification or procedure.
Remember that if your goal is to save fuel rather than getting good MPG then taking a shorter, slower route will probably use less fuel. With me it is a hobby, and I'll drive a farther route if it will give me better roads to get better FE.
1. Drive Slow;
At higher speeds, overcoming aerodynamic drag becomes the main use of your fuel. At all speeds you have friction from rolling resistance, bearings, and internal engine friction. A goal of minimizing all drag or friction is a noble one.
2. Use the highest gear possible;
Horse power is torque times rpm. Engine load (i.e. fuel burn) is usually related to horse power. In theory any given horse power will use a set amount of fuel. However, the internal friction of the engine, (piston rings and cam shaft gears or belt, and internal bearings) is related to rpm. In aircraft in the old days, it was accepted practice to lower the rpm (by coarsening the propeller pitch) and increase the throttle setting, which increases the BMEP (Brake Mean Effective Pressure), while reducing the rpm and therefore resulting in less internal friction of the engine for similar horse power.
I shift from first to 3rd at very low speed, and shift from 3rd to 5th at about 35 MPH.
3. Brakes are for parking only;
Every time you use the brakes, you convert the vehicles kinetic energy into heat of the brakes. That energy was initially obtained by burning fuel during acceleration. Idealy you would coast, so as to stop at a red light or stop sign without use of brakes. Other traffic, and city driving means that you are often using the brakes. I am always a bit sad when I apply brakes. It is always a waste.
4. Never idle;
Idling an engine is a waste of fuel. I have a manual transmission car, and a button on the gear shift lever that cuts power to my fuel injector. If I am faced with more than a 10 second idle, I just stop the engine. My car idles at .13 GPH. With the fan on high it idles at .15 GPH. My DRL increase my idle fuel flow by .02 GPH, headlights by .03, and the A/C raises my fuel flow by .17 GPH. In heavy traffic where idling is necessary, turn off all electrics. I usually drive with the hand brake on the first click to turn off the DRLs. I also crack the window open slightly to be able to keep the fan off. Never use the A/C!
If you have power steering or power brakes, then you may need to keep the engine at idle. If you are coasting with the engine running, then your instantaneous MPG is related to speed.
5. Aerodynamics at the rear of the car is more important than at the front;
How a vehicle cuts the air is not near as important as how it leaves the air behind it. More of a car's body drag is caused by the turbulence behind the car, than from the pressure in front of it. A large percentage of the total aerodynamic drag of an unmodified car is caused underneath the car as opposed to the top and sides. A front air dam helped my FE by a significant amount. One advantage of an aerodynamic improvement is that it is always there, and everyone driving the car will benefit.
6. Scangage is your friend;
The only way to reliably tell if a driving procedure, body modification, or engine enhancement is really working is with something like the Scangage II. It is also a good warning of improper procedures, or bad driving practices. A Scangage is also best utilized when it has been calibrated after a few fill ups. After many fill ups mine is accurate to within a tenth of a gallon at fill ups. My actual milage and fuel used, versus the Scangage readings are very close. An un-calibrated Scangage will also give improper readings on things like instantaneous fuel flow, or mpg. Giving a high percentage during fill ups will give you a very nice MPG reading, but it is lying. If you quote a good MPG from a Scangage reading, and it hasn't been calibrated, then you are lying. :-)
7. Always Pulse and Glide.
If your car had zero rolling resistance, and no aerodynamic drag, and was on a perfect road, then after you accelerate to a given speed and shut the engine off, you would coast for ever. Infinite MPG. Any speed lost going up a hill would be regained going down the next.
The cars in the unlimited Shell Eco-Marathon get over 9000 MPG. (
Shell Eco-Marathon winner averages 9737mpg ) They are supper efficient and use brief acceleration followed by a very long coast.
Traffic makes P&G very difficult without inconveniencing other road users. I drive country roads and use the emergency lane.
8. Weight of the vehicle makes no difference.
Because in a perfect P&G world you are not losing energy, the weight of the vehicle doesn't matter. The kinetic energy you build up once during acceleration will stay with you for ever. But, if you are in an environment where you need to start and stop, the use of brakes will eat the extra energy you applied to accelerate the extra mass. That energy source is the fuel.
Also in the real world we have rolling friction that increases with vehicle weight.
9. Aerodynamic drag is both drag coefficient and frontal area;
So a super sleek 18 wheeler that has a very low drag coefficient will still get poor fuel economy because of the large frontal area it presents. I have done many aerodynamic mods to my RV similar to the ones done to my Metro, and I feel them to be a waste of time.
Here is a list of mods and procedures done to my driving, and vehicles in order of amount of benefit received from each.
Driving slow.
Pulse and Glide.
Injector cut off switch. (Allows engine off P&G)
ScanGage 11.
Front air dam.
Tire pressure at 44 PSI.
Rear wheel skirts.
Removing windshield wipers.
Minimizing electric loads.
Front Air grill block.
Remove passenger side rear view mirror.
While all of these suggestions are out of my head, they are the result of inputs into my head by many good and knowledgable people on the ecomodder web forum. Most concepts I mention have been over simplified. A book could be written on the various topics I have mentioned. Experts can discuss and argue for hours, debating each nuance of what I say here. This guide is not for experts to argue about, it is to help get Newbies started in the right directions. If you know (not just think) and can prove, that something I have said is false, then please advise me and help me learn more about the art of getting good fuel economy.
Kgwedi