I can normally run a 7.8 final drive for carpet. Our track is small though, most of the guys here are running 8.0-8.3 final drive ratios.
For 1/12 just lighten up the springs. I'm pretty sure we put purple/red on it initailly, but I will usually run red/green on my 1/12 and start with full face brushes.
-Korey