MPG is a bad metric because someone going from 10 to 12 mpg, a 2 mpg or 20% increase, will save the same amount of fuel as one going from 30 to 60 mpg, a 30 mpg or 50% increase. Much better to express in terms of gallons per 100 miles since distance traveled tends to be constant and the gallons vary. In that case change from 10 to 12 becomes 10 to 8 1/3 and the change from 30 to 60 becomes 3 1/3 to 1 2/3, or 1 2/3 for each.

The mpg metric makes it appear that Hybrids make the most sense for small cars, but in reality the most fuel would be saved in large vehicles (think taxis, local delivery trucks, busses).

And if you want an even better metric, try gallons/year/person. That's what really counts!

(And in the spirit of the orignal post, dollars/year/person).