Damn. Super clean. A side-effect of the car's engine hum (coincidental or not) was that staying at top speed gave a nice droning base note in the music's key... Hmm... interesting subliminal feedback telling the player to stay as close to the "harmonizing speed"....
What was I saying? Oh, yeah. Anyway, potential music racing game (for you to make out of this nice framework that you've made for yourself!) aside, nice work! The sprites fit nicely and it feels quite cohesive overall.