under the help tab, if you go to upgrades and look at rollover bonus...it says "This upgrade will "incrase" your multiplier...", but i think you mean: "This upgrade will "increase" your multiplier..." instead.
You should add a way for the players to make a custom terrain, so we can test out cars. You probably shouldn't add coins though. Then people would make flat terrain to get a lot of money. + if you agree.