In 1939, a group of scientists at Shell’s Wood River Research lab in Wood River, Illinois, got in a friendly argument as to whose car got the best gas mileage. They finally decided that the only way to settle the argument was to have a contest where everyone got a gallon of gasoline and whoever went the furthest won.
They called it a mileage marathon and continued the tradition until the lab closed in 1975. In later years they added modified and unlimited divisions to the stock rules. The unlimited allowed any modification to the vehicle except that the vehicle had to weigh as much as the original vehicle and had to have an engine that was originally available in the vehicle.
Over the years some employees went a little crazy in their modifications. Being a leading crazy, I set the world record in 1973 at 376.59 miles per gallon, which was listed in the Guinness Book of World Records.
So, what does that have to do with general aviation? The principals for improving gas mileage in a car also apply to aircraft. The main points are optimizing engine operation, minimizing weight, reducing aerodynamic drag, and improving operation parameters.
Optimizing Engine Operation
On the engine operation, the main thing is to make sure that the engine is up to par and all adjustments are correct.
The first item, of course, is timing and mag operation. If the timing is retarded, you will lose a significant amount of efficiency.
The same is true of a fouled spark plug. If you have a fouled plug, the cylinder may still be fired by the second spark plug, but the fouled spark plug will effectively retard the timing and reduce fuel economy.
This is fairly common on low compression 80/87 engines operated on 100LL. The spark plugs core tips in these engines run at a lower temperature, which does not burn off the lead. This results in lead deposits on the core, which can ground the spark before it can jump the gap.
If you have an 80/87 engine and run 100LL, you need to take steps to ensure that the engine is not misfiring or mal firing.
Another consideration for the engine is good compression on all cylinders. A leaking valve or broke ring will have a definite negative effect on fuel economy.
One of the other considerations is using multi-grade oil, which should improve your fuel economy 3% to 4%.
Excess weight does not just refer to the pilot, but also to unused clutter.
Another thing is accessories, such as air conditioning. I know air conditioning is nice, but it costs you in three ways. First is when you buy the plane, second is from the drag on the engine, and third is due the increased weight of your aircraft.
Even electrical accessories are not free because they increase the load on your engine to recharge the batteries.
The main concern with weight is loading your aircraft with the proper CG. If your plane is improperly loaded, it will take additional power from the engine to maintain safe level flight.
Reducing Aerodynamic Drag
Aerodynamic drag is mainly ensuring such things as proper door gaps, etc. Other possibilities are wheel pants or STC mods that can smooth out your aerodynamic posture.
And, of course, speed is the biggest consideration. I know we fly to get someplace quickly. But running at 65% power will reduce fuel burn significantly over 70%.
Improving Operation Parameters
Finally, there is operation techniques. I have mentioned speed, but there is also reduced idle time, flying at constant speed and altitude, plus other items.
But the big factor here is leaning. Leaning can offer the biggest improvement, but also offers the biggest risk.
In an engine, the chemically balanced air fuel ratio is called stoichiometric. At this ratio, all of the fuel and air balance out and is usually around 14.7 pounds of air to one pound of fuel.
But this results in maximum combustion temperature and can lead to a burnt cylinder assembly.
The best power is usually around 10% rich of stoichiometric, which is needed for takeoff and climb out.
Unfortunately, the best fuel economy is about 10% on the lean side. Theoretically, we should operate the engine on the lean side during cruise.
The problem is that the fuel systems on aircraft engines are not perfect, so there is usually a wide variety of air fuel ratio for the different cylinders under varying conditions.
In the automotive world, I believe almost every car produced in the US since 1968 has operated lean of peak during cruise conditions, but they are now all liquid cooled, so burning cylinders is not a problem.
In aircraft, operating just one cylinder at peak can lead to a problem. If you choose to do that, you should have multi-point temperature read outs so that you can monitor all cylinders and not overheat any of them.
Will all this work make a huge difference in your fuel bills? Maybe not, but it should help.