story
x4-3*x3+x2-x1 --> x4-3*x3+3*x2-x1
Good luck with the project.
PS: Usually tweaking the parameters to select the opponents/ghost and make the game fun (that is what is important) is hard. (More cars? Less cars? Use the last time of the player? Use the average time of the player?) So after you solve the problem, a post-mortem may be an interesting blog post to summit here.
PS2: another typo in my first comment: defectively -> effectively