Having had all of my progress eaten by the PlayStation 3 problems, I am certainly down with a little creative cheating. All you have to do is place a fan in the game's Toy Box mode, spawn a stunt buggy (the big wheels help) and inch up to the fan ever-so-carefully. Do it right, and you'll be suspended in the air, where you can spin your way to stunt feats. Do this for a few minutes, land, and enjoy the results.
Here's me and Mike, rapidly jumping from level 5 to 8 (I went from 1 to 5 before I hit record).
The only problem with the process is it is extremely hard, nigh-impossible to get it working with the characters from Cars, since they've got their own short tires and can't ride another car in an E-rated game.
Update: Spinning as a car works!
It just doesn't give you the experience boost, because you aren't IN a vehicle.