1. Buy a Peugeot 908. You will want a La Mans car to complete the game anyway.
2. Buy a High RPM turbo.
3. Go in to car setup select tab B or C so its a quick tune setting, install the turbo, Lift all down force to lowest setting, and run the transmission up to the highest mph. Stretch the final drive out a bit so the car will hit 300/310mph.
4. Jump in to the "Like the wind" event and start the X circuit race.
The car will cruse at 280mph and in a draft it will hit 300mph. You will be in front of the pack before the first turn with room to spare. Each race takes just over 4min and makes you 50k before log in bonus.
Happy grinding.