Hey,
Nice little racing game! I like the idea and implementation of how you increase the speed! Fits well with the theme! Good job on different levels as well! Just an idea, maybe there could be some obstacles to make the race harder?
Unfortunately I couldn't try out the multiplayer but sure that would have been a lot more fun try out!
Good job!
ps. Difficulties did not seem to have much effect on the game play, do they?