I run a fairly large track that has lots of technical areas and jumps. In my nitro converted 3.0 I'm running a 17t pinion on my tekin 1900kv with a 45t spur (metal). My motor temps at maybe 100* in ten minutes. The battery is never warm. I have plenty of power in the infield and can run with the 2650kv motors down the straight.
I tried running a 13 and a 15t on the same track and after 5 minutes the battery was really hot to touch and the motor was also hot - when I geared up everything balanced at 16t and 17t pinion.
My thought is the motor was more consistently at full RPM and the battery was discharging at its max rate through the ESC. With the higher tooth pinion the torque was awesome and the temps dropped to almost nothing.