Having a suitable gear ratio helps, usually the le mans cars are geared too long. I assume you are in a le mans car like the Sauber?
I found it hard going in the 95 le mans McLaren with engine upgrade. But I'm still trying to get used to the new physics, I'm probably 5 seconds off my usual pace especially in race cars.
You should probably get a LMP so you have something for S class as well. Minolta, Sauber C9, 787B, R18, Deltawing, Speed8, Gt-One, R89C or R92CP should suffice.