What am I doing wrong

Not necesarily starscream. In both my revo and slash, reducing pinion size reduces temp. It is easier for the motor to push a car around if the gearing is lower (lower temps), unfortunatley that comes at a cost; speed. The motor will ALWAYS spin at the same speed with it's correlating throttle position, no matter what the gearing. If you increase pinion size, it is harder for the motor to move a car (more heat) but you get higher speeds.