You will probably want to start with a gear ratio around 6.0
That number is derived from the following
Teeth in spur gear, divided by teeth in Pinion, times the internal gear ratio.
I don't know off hand what the gear ratio is for your car, but for instance with a TC3.
2.5 internal ratio.
30 tooth pinion
That give 6.0 exactly.
Start with something like that and gear up or down depend on how the car feels, ie... accelerates and where it tops out on the straight. You want it to be toped out about 2/3 down the straight probably.