This game is great! It's fun and challenging. Great work.
Here are some suggestions that I think would greatly increase this game's appeal.
Particle effects on the car tires. Some sort of penalty when you go off track, grass should slow you down. Button configurations... Allow the player to change their controls. You need to implement some sort of menu for that. Turning sound effects at speed. This would give the player some sort of indication that they are too fast for the turn. Also, the isometric view allows the player to not see corners at speed. Some better indication on the track would be nice.