Well, you could actually do okay, Jeff. IF enough people are interested, the first method I'd use for classing would be simply displacement.

Probably under 1.6 liter, then 1.61-2.2, 2.21-2.8, and 2.81 and up. Something like that for starters. So, your 30 mpg would be up against cars with 2.2-2.8 liters.

(of course, the truly "green" way to class would be on the carrying capacity of the vehicle)