Need some help here real quick.

I'm going to run 17.5 class at Motorama just as a second class but I have never run a 17.5 and have no clue what to gear them at. If possible, I wanna keep a 81 spur on but I don't know if you can hit the right ratio.

Can any of you guys who run 17.5 (or have run it) help me out here because I need to buy a couple pinions. I will be using a LRP SXX v2 and probably a loaner Tekin motor.

Thanks!