This is for having 18"s or larger on cars that normally cannot have them (i.e. The Miata can only have up to 17"). Money, Level 2 and/or 3 Rims, you also need to be far enough in the game to have a Trade In car that has big wheels is required for this Glitch.
1. First find a car that can have larger rims than your current car (Miata 17" - RSX 19" )
2. Select Underground mode, go to Customize Ride> Trade In> Now trade your car (Miata) in for the car you found earlier (RSX).
3. Go back to Cutomize Ride>Visual>Rims and reinstall the rims you currently have (or upgrade to bigger and better things).
4. Now go back and trade in this car for your old car.
5. Enjoy your spiffy big rims that your car shouldn't have!