I run anywhere from 100k to 500k in the center diff but most of the time its 200k or 300k, sometimes ill run 500k is the track is calling a tire like slicks but for dirtwebs or ions/electrons i am around 300k. then for dirt ill run 200k or even 100k if its really loose outdoor. I did find that of course the lower you go the easier the car is to drive, the only issue is your fast laps will be down time wise I try to find a good middle ground, easy to drive but able to run fast enough laps to keep up.