Cool game! An unconventional way to propel the car forward. The game worked well, I only found one bug, that happened when going really fast, and the car just jumped super high into the air and landed with the front into the ground, effectively stopping the car, but that was probably unity physics fault, not yours.
And after like 10000m there's really not anything you can do other than being in the middle of the road, because turning a little may cause the car to go into a tree that suddenly appeared because of how fast you're going, but I think it's really cool that you can go that fast.
Great entry! :)