For those of you not acquainted with Seth Godin I highly recommend his blog if you want to read the musings of a really smart guy and pick up some knowledge nuggets along the way. Take his latest post, for example, on how bad we humans tend to be a math unless we train ourselves to be otherwise. Go ahead - read it, then come back here for the reveal.
If you still don't quite see why he's right and don't want to do the math, I've done it for you here. Don't worry: it's not hard, and there's no test later. No doctor will call. Operators are standing by.
The first point to consider is that it doesn't matter whether we're talking about a thousand cars or a single car, so in this example we'll assume one of each vehicle that drives 10,000 miles in a given year.
At 10mpg the stock Suburban will require 1,000 gallons of gas (10000 / 10). Improving its mileage from 10 to 13 mpg means it will only need ~769 gallons for that same distance, so that small improvement will mean a savings of about 231 gallons of gas.
The Prius, at 50mpg, will need 200 gallons of gas to drive those 10000 miles (10000 / 50).
Doubling the mileage of the Prius to 100mpg will mean it only needs 100 gallons to drive 10,000 miles (10000 / 1000) for a savings of 100 gallons of gas.
231 is much more than 100. So, as you can see improving the efficiency of a Suburban from 10mpg to 13mpg yields more than twice the savings (in gas, money and emissions) of doubling the mileage of a much-more-efficient Prius. You just can't escape the law of diminishing returns. As the great philosopher Barbie put it "Math is Tough".