I did a quick check at TireRack.com and found the calculation for tire diameter based on the tire size, no snooping required. :lol: I had been under the impression that the different rear gears were to compensate for overall diameter differences, but that doesn't appear to be the case here, although a 1/2 inch may make a bigger difference than I know. Rather, the desired gear for maximum performance is the 3.92 and to prevent potential shredding of the rear tires during fast starts they need to install 20" wheels with Z rated rubber. By keeping the 3.73 rear for the 18" wheels, they're safe with lower rated, taller rubber and slightly slower starts. I'm guessing that the 6 speed R/T with the 3.73 rear will get a little better mileage than the 3.92 too. This is all speculation mind you, and someone here with more experience than me is welcome to throw the BS flag.