How about something reasonable, like cost to operate per mile.
When you add in the abuse that you put on a lightweight vehicle when it goes more than 12,000 miles a year, the MPG goes out the window.
What abuse would that be?? The wife's Toyota Corolla has about 100 mile/day put on it (five day work week). It's coming up on 50,000 miles, and the gas mileage is still about 39.7 mpg (actually up a bit from showroom new mileage).