The few times I used mine on 6s (
http://au.youtube.com/watch?v=1a2zmUbG8Ic) I just used my normal 4s racing gearing (~35mph), so it ends up being about 50% more top speed, which was enough for me. After 10 minutes it did get about 30 degrees hotter than usual 4s running, but that's still only 150f.