Hey Jason, I'm back, looking for more ways to chop up the wonderful mileage DB for our edification :-)

Could you normalize a model's average mpg to miles driven ?
E.g., if one Prius had been driven 100 miles at 50 mpg, and the second Prius 200 miles at 60 mpg, the normalized mean for the two car group would be 170/3 mpg. Doing so would give more weight to cars that had been on the road longer, and so emphasize driver experience and seasonal averaging.