ArmorGames, How would you feel if I asked you if you could make a second one. Epic Combo! 2. A second one could have more upgrades, different backgrounds, more modes, a speed button for people who hate having to wait for the combo to go up.
I agree with what thesurviver said: "my suggestion for a upgrade system would be to split the current ones into sub-upgrades... for instance, boost could be split into "Capacity"/"Duration"/"Power" and then Engine could be split into "Horsepower(max speed)"/"Acceleration (how fast it gets to max speed)", Tires can be split into "Drift"/"Grip"/"Duration", The fuel can be split to "Duration"/"Capacity"/"filter"(the higher the level, the more pure and efficient the fuel is making the car faster) :) these are just a few of the possible ideas for upgrades. + if you agree :P"