This are just my guesses, but here I go:
N100- S-FR
N200- 86
N300- X-Bow
N400- 911
N500- 911
N600- 911
N700- 911
N800- La Ferrari
N900- La Ferrari
N100- La Ferrari
Gr. 4- Megane Trophy for slower tracks, Veyron for faster tracks
Gr. 3- Varies per track
Gr. 2- Varies per track
Gr. 1- Porsche 919
Gr. B- Varies per track