The FDR with the HPI on the stock 23t pinion would be 8.33. 90 spur and 23pinion with a trasmission of 2.13 vs. the tc5 at 2.0. Currently my HPI is running a FDR of 7.66 with the 25t pinion i have installed.
Just got all my gears in and mounted the motor with a 17t pinion. That will start my FDR @ 10.23 and will be using this as my starting point. Before I run the tests though, I am going to take your advice and head to the track this weekend.
I'll post back and let you kow what I find.