Unfortunately, there is no way to change display settings in asphalt 8 if you are running it on windows PC or Windows phone. But if you download asphalt 8 for Windows outside of store, u will be able to get that option.
It is actually not possible to completely finish the game. That is not the goal of the game. You can, however, get rather far without spending any dollar. The most important thing is that you have fun, making progress is great too but not really necessary.
Best Answer
go to the main screen and go to settings> sound and display> speed unit, and press MP/H