[TR65]

Just some more info.

The tires, the aero, and the inertia losses all together account for only about 30% of the fuel use. Knock each down by 10% and you would only increase mileage by 3%. Actually I think our trucks have considerably more aero losses than the numbers used for modern cars.

Anyway, most of the losses come from thermodynamics, pumping losses, engine friction losses, transmission/diff friction losses, and accessories (P/S, AC, Alt ...). All of these losses are reduced by lowering the rpms. Every cylinder has to fire every two turns of the crank and it takes fuel to fire.

Here is a rough distribution of where the fuel goes.

TR