how about simply using the percent over the new EPA rating
I second this idea... although my vote might not have much weight, since I don't know if I will even be participating if/when the challenge gets going again.
One caveat about % over EPA, is someone impartial should check the EPA ratings for the vehicles participating, and that could be difficult for vehicles before 1985. Does anyone participate with a vehicle before 1985?
My Regal is a 1981, but it did take some time to find the EPA numbers. Another member kindly supplied them for me. My primary "competition" vehicle would be my 1998 4wd pickup though. I would imagine if one encountered a pre-85 vehicle when auditing that they could just let the numbers stand if they sounded reasonable. If someone was was competing with a 1978 Datsun 210 and they had it "rated" at 15/20/18, and their actual mileage is in the 30's and 40's then you know something is up.
EDIT: I was just thinking... Maybe have separate classes for types.. Like maybe 3 classes: 4 cyl & under, 5 & 6 cyl, 8cyl & up.