15t is good starting point. I'll be gearing my Mugen 17/46 for roughly 43 mph, running a nue 1515 1y, the motor that the castle motor is based of of. 2200 kv on 14.8v. I have a 15 if I need to gear down, but temp wise it should be fine. I ran that same motor in a converted revo. It a lighter buggy it should be just silly.