Assuming you mean 72T spur as I don't see a 75T spur option for yokomo on amain.

In general when I don't have a starting point at a new track, look to target an FDR of 7.5 on 2wd buggy and then adjust gearing based on where I am topping out down the longest straight, relative to temps. that in general has worked well since you can have some variances on how the motors best respond to gearing.

In general on a smaller indoor track I have seen with a 72T spur a range from 27 - 31T for spur usage, depending on motor (torque versus something more RPM based) and track conditions (open versus tight, etc)