i like how our slant six club has it set up
AA-Rails
A-2000-2299lbs
b-2300-2699lbs
c-2700-3199
d-3200 and heavier
then
/A-Alky
/G-gas only
/N Nitious
/U unlimited (fuel, blowers, etc)
if you wanted to factor in a scaled weight and CI you could to even it some more