How to tune Mine Cooper 65 to S class?

I max out everything but the best I can get to is A class. I see guys with Tune setups that go to R class. Do I need to tune it?

Probably need to do some sort of engine conversion. You can find it all the way over to right in the Upgrade menu.

Tuning won’t change the PI rating and the class. Try looking at the engine, drivetrain, and aspiration conversions.