Juiced Cheats

Buy a car and let one of your crewmembers drive it in a race three times (if he is winning them) or four times (if he is not winning). The fastest way is a 2-lap circuit race on your own track.

To train your crewmembers, simply get them cars in the same class and make a race on your racetrack (3vs3 for example), choose circuit and 12 laps and watch them race. Repeat a few times and their stats get maxed in a reasonable time.