A good start, could use a lot of improving!
The car has too many straight lines and not enough bumps; its main color might be different from the sky; one more highlight color might be useful.
The black outlines are boring. Use selective outline, or just get rid of'em.
Cool piece, looks like it could use some polishing though.
Kinda reminds me of this: http://pixeljoint.com/pixelart/34772.htm
I like this :D
The perspective of the wheel is a little off, and you could always do a little anti aliasing (Even with the colours you currently have)