Tried one more....69 Charger. This was similar to the other builds, but I put a rear wing on it. I went ahead and ran the same toon for all the tracks, with plenty of downforce. I may have lost a little on Copperhead, but I more than made up for it in Russel's Viper.
Copperhead 118.468
Boomslang 111.054
Russel's 93.878.
If we're allowed to run different toons, I suspect there's some time to be gained on copperhead. I'm not sure how much of this improvement is the tightening of the nut behind the wheel, though.