[brucepick]

In my last post here I wrote "limit it's revolutions". I might have better said, "reduce its revolutions".

A good amount of energy / fuel is needed just to turn the engine. That's why we go to taller gearing, and also kill it and coast so we can eliminate as many engine revolutions as possible. You'd be surprised at how much energy is used just to make it turn.

Mine needs approx .155 gal/hour at 670 rpm idle, and that's with the alternator disabled and replaced with a deep cycle battery. (With alternator charging, it's more like 17 gph.) Interestingly, at 2000 rpm which is just about exactly 3x idle, it needs about .47 gph. In other words, no change in fuel qty per revolution even at 3x the speed, it's just about linear, so far as I can tell.

Doing some more math, I found that at 2000 rpm at about 50 mph and 50 mpg (yes, my car does that, sorry), the engine is using roughly HALF its fuel just to keep the engine turning! The other half of the fuel is available to move the car forward!

So, you can reduce your total engine revolutions by gearing it taller, or by coasting with engine off (or idling is also good), or even a "rubber overdrive" aka bigger diameter tires. Bigger tires = more rotating mass, there are penalties for that so taller gearing is better. The less you turn the engine, the higher your overall mpg will be.