There is no "tweaking" aside from changing tires, and the base tire models are in standard lettering... "S2", "R3", etc.
The second option at the Race menu changes the physics style. The top one is arcade, the bottom is the new physics engine, Simulation or whatever you want to call it.
What I'd like to see myself is a translation of the control screen. I remap my GT controls, and it's been rather a pain to figure out what's what on that screen.
