15 sounds real close to me, that's about 40 to 41mph depending on tire compounds. You can sand a little off of the center differential brace and put on a 14t pinion, that would drop your top speed about 2.5mph. For me if 15t is running good without any heat issues I would stick with it. I normally like to run a little bit over geared, for my driving style it works for me. I've noticed on this forum most of the Tekno drivers are racing with real tall gearing. Like 18t pinions with 4300kv motors. Got to be rockets on the straights, all about control and serious talent imo.