Sorry, it's a bug.
First of all, this was submitted for a game jam. The game would automatically give you coins depending on your top speed, but it would break the math of the game (giving you too much money too quickly) so I had to disable it and deliver it as is before the game jam's deadline. I'll try to fix it but I'm going to have to wait for the voting period of the jam to update the game.
Sorry for the inconvenience and thanks for playing!