Hi, as a big fan of the original Enduro for A2600, I am very excited that you are making a version for the C64. I realize this is still a WIP, so I hope you won't mind a few suggestions :)
First, I think the player car sprite is a bit too big and clunky. Second, I think the horizon is too close. Some car sound effects would also greatly enhance the game, I think. I really love the colours though - they are gorgeous!