> And that'd spur people to get fuel efficient cars: if everyone switched from an SUV to a Prius or Fit, the amount of oil saved is several times the amount used in agriculture.

Not so fast. Building cars costs significant energy. The result can easily be more energy consumption.

Not to mention that switching doesn't necessarily work. In most cases, the only realistic alternative to "big car" is multiple vehicle even if the big vehicle's capacity goes unused most of the time. Owning two vehicles, one big for "those days" and a small one for the rest of the time is usually more expensive than owning just the big one unless the small one is very cheap.

I've posted the arithmetic. If the big car gets 15-20mpg, the small one has to cost less than $4k even if it gets 50mpg. (My arithmetic was conservative because I just looked at purchase and gas, which ignore many of the costs associated with a second car.) If the big car gets 25, it's almost impossible to save money with an additional vehicle.

And no, renting doesn't work out significantly better.