So close....
This was a very nice game, it was nice that you had some orginal voice acting in it.
There are a couple points i think could improve the game...
First the controls, why was there no WASD option considering you have to use space, 1-5 and control, those can all be easily reached around the WASD keys, but from the arrow keys you have to move you hand off them to reach most of those keys.
The upgrade menu wasnt especially obvious, i didnt see it at first, and then took me a couple seconds to realise it had several screens. Personally i would have had an "upgrade" button in the middle of the screen after each level, and maybe had a tabbed based interface for the different sections on the upgrade menu.
Lastly the collision detection was a bit off, since it was based around the entire sprite, rather than the chasis/perspective of the car. So if you clipped the roof of another car that counted as a colision.
Otherwise quite an enjoyable game, with some high priduction levels in places.