[Christ]

Downsizing your engine is only effective if your current engine is capable of making too much power for the RPM/vehicle speed you need to be at.

The mileage gains aren't all that high, that I can tell, either, because the only thing you're really lessening is the internal resistance of the engine, pumping losses and frictional losses. Sure, that accounts for a large bit of engine power lost, but there is much lower fruit ready to be picked.