So what car and race is everyone running to quickly get some level ups? Goliath Circuit I would assume? Not sure what car gives what boosts.
There are currently 3 cars with the XP boost:
Jaguar F-Type P7
Ford Mustang '69
Subaru SBZ
The Sub is tuned as a drift car though and the Jag is one class higher than the Mustang, so the Jag would be the best choice for getting XP since it's the fastest.