Well that depends on what the final drive ratio you're trying to achieve. Depends on the track you're running too. What motor do you have in it? Is it the TC or the earlier model because the have different "transmission" ratios. In my TC I'm currently running the 100 spur and a 50 pinion. That put me at a 4.7 FDR. With a 13.5 you could go as low as 4.2-4.4 without any problem
Just in case this doesn't help I know that this link will. It made my life a lot easier.
Gear chart Just punch in your car info. If you have the TC you'll have to manually enter the trans ratio which is 2.167 to get the chart you need.