I played up to the ice mission, the gameplay itself was good, the driving was reasonably responsive and felt satifying. The art style looks great, simplistic but really gets across that race car feeling.
All of the levels at the start seemed quite easy, not sure if thats just to encourage you to replay them for a better rank, or just cause they get harder as they go on? Not sure how many levels total are in this, but a bit more feeling of challenge would be good, maybe more obstacles as well as the winding tracks?
Perhaps adding some kind of minimum pass time show clearly too would help motivate the player to get faster.
The audio was quite repetitive (though I appreciate its not easy to find good music that loops at short notice) and a few of the sounds, like the win sound felt slightly out of place.
My main issue was just that the inclusion of the theme didn't feel particularly significant. In race you're on a timer anyway so counting down wasn't too unique, and while I think the concept of having the different 'gears' have usage timers is interesting, it doesn't really make sense, and I only had that timer run out once, which again, because of the low difficult didn't matter.
All in all a very fun proof of concept, with some great art and great ideas, keep it up!