Actually, the single most effective thing we could do to reduce fuel usage, short of getting rid of the SUVs, would be to reinstate and enforce the national 55 mph speed limit (well, if by "we" you mean "the US"
) However, I don't know too many people in favor of that.. even though I travel at that speed most of the time, I don't like laws. But, it would be about a 10% increase in overall efficiency, and not require people to change their vehicles. Just their habits.