On 4s with the 2250kv motor and 25/63 on stock tires you'll be at 42mph, factor in some tire ballooning and you're over 45mph. Personally I'd start here with your motor to make sure temps are good before gearing up any higher. Pinions should in 25t mod1 are available lots of places, Losi makes one, so does Robinson Racing, and many others. All my pinions are either. Robinson or RC-Monster but I have a couple AE and they make some nice mod1. Not sure if AE makes a 25t though.