To decrease lap times, try driving with Steering Assist and Diff Accel lower than 100% and 30% respectively. It will be more difficult to control your car until you get used to lower settings but your lap times will gradually decrease. When lowering Diff Accel, be careful not to push the gas too much too quickly!

I am currently using 80-20 and have noticed approximately a 0.5-1 second difference. I only race with manual shifting.

Steering Assist and Diff Accel are only adjustable for the Modified Series... at least for now.