Originally Posted by gticlay

For 13.5 a 7.0 FDR is a good starting place. So on the XB4 if you used a 78 spur with a 28 pinion combined with the transmission ratio of 2.5, you would have a 6.96 FDR.

I am just getting my XB4 together but we have a really fun 13.5 scene locally and I was running 28/84 on my 22-4 which has a 2.3 TR, giving me a 6.9 with 40* timing and the motor was barely heating up. I could have easily have gone up a tooth or two if needed but the car was nice and fast.

So that's what I would start with. If you want to use the stock spur, it's an 81 so a 29 pinion would give you a 6.98