Suggestion -- Make a stats page unlockable (able to purchase) via Upgrades. It could show profit +/- for each Investment (as others have mentioned) and so on...
#1 Flaw - Test track vs. Race track...completely diff handling and times for the car. WTF?
It needs a slight tune-up to be a MUCH better game, but it still has potential. Also, if I don't get the right parts in the right order, there's no way to sell and do the trial/error. So new players, guess away, start over if you're wrong. Ppl that are having trouble...learn to shift differently :)