I would start at a 2.0" rollout and go up or down from there. Just run the car for about 4 minutes and check the temp on the motor (I just do it by feel). If it's very hot to the touch gear down and try again.

Other than that, play with the gear to get the fastest laptimes. The motor temp is nothing more than a safety factor and it will shut down if you overgear it (saving the motor).