I have the same question, recently back in onroad, same car, same speedo! Don't take my word for this but through searching the tekin and tc5 threads, it seems most using a 17.5 with ramping here is gearing at about 4.0 final!! Internal on TC5 is 2.0:1 so it's real easy to calculate in your head... Just double the pinion for the spur and POW!! 4.0:1!! With that said, a temp gun is always best.
I think there is a lot of factors now that need to be worked out at the track such as timing and ramping and c ratings so I'm not sure what these guys and gals could tell us but starting points are where I need help.