I have a feeling that if SUV's were suddenly all replaced by smaller cars, then the little USA bar on that graph showing how much oil we consume would only come down a fraction of a millimeter, if that. The difference between mpg on cars and SUV's isn't really THAT great. And I don't think it would impact the oil consumption too much. let me do the math. (and somebody please double check it)

I'll use Biscuitjam's figures: that there are 16 million SUV's out of 200 million vehicles. They average 21 mpg, as opposed to the car's average of 28. Let's say the average auto is driven 10,000 miles/year.

16,000,000 x 10,000 = 160,000,000,000 miles/year
160,000,000,000 / 21 = approximately 7.6 billion gallons/year

So SUV's use about 7.6 billion gallons of gas in 1 year. If those SUV's got 28 mpg then...

160,000,000,000 / 28 = approximately 5.7 billion gallons/year

So then they'd only use 5.7 billion gallons of gas in one year instead of 7.6. That is a difference of 1.9 billion gallons of gas. According to discover.com, typically 19 gallons of gasoline can be produced from 1 barrel of oil. So...

1,900,000,000 / 19 = 100,000,000

So 100 million barrels of oil would be saved each year if all SUV's in the US got 28 mpg instead of 21. That sounds like a lot. It sounds like a lot until you look at that graph that someone posted above and see that the US uses 300 million barrels a day.

EDIT: I actually meant 900 million barrels a day, but that seems to be incorrect. see my next two posts. It should be more like 20 million/day.