[Dane-ger]

Just because a modification increases the engine's MAXIMUM POSSIBLE power output doesn't mean that it decreases fuel economy. Underdrive pulleys, lightweight flywheels, free flow exhaust systems, etc...make the engine more efficient because they decrease the drag on the engine...so it takes less work (work...as in calories, joules, foot pounds) to turn the engine...therefore LESS fuel is used. Its true that these types of modifications increase horsepower...but you only see that increased horsepower if you have a lead foot...and even then the engine operates at maximum thermal efficiency at full throttle since the cylinder pressure is maximized (not that maximum FE is achieved by maximum power at max thermal efficiency).

Actually, if I were you I would use the smallest engine displacement possible. The best way to do this would probably be to destroke it (if the corresponding piston and rods to maintain piston to head clearance are available) and either turbocharge it or go with a higher compression ratio, but only turbocharge if you can significantly reduce the displacement....and even then use only a few psi of boost so you don't start guzzling gas. That would maximize your thermal efficiency. Turbo engines have high thermal efficiency because they take waste energy (exhaust heat) and convert it into useful work. High compression engines have higher efficiency because the increased cylinder pressure maximizes the PV (pressure-volume) work extracted from burning a given amount of fuel. A small turbo engine uses less fuel than a large NA engine of the same power output. Same with a high compression engine. Diesel would be best though...and turbodiesel is the best of both worlds...maximizing cylinder pressure at a given power output with both high compression and boost, and harnessing waste energy with the turbo.

You could also use themal barrier coatings in your combustion chambers and piston heads...so there would be less heat loss into the cooling system. Using a thermostat that raises you engine temperature would also increase efficiency...my grandpa was a mech engineer and did this big study on thermal efficiency when he was in school...always said engines run more efficiently at higher temperatures.

I guess the whole point I'm trying to make is that to get maximum fuel economy at a given power output, you want to minimize engine drag and maximize thermal efficiency. Coincidently...this will also increase your full throttle horsepower if you ever choose to go there.