My friend Bill is answering a question about what is a good FDR for a 21.5 motor. My answer is, it depends on the motor. I've run between 2.9 (by accident & I was about as fast as Bill was that day) and 3.6 depending on the power band off the motor. High torque/low rpm motors like numerically smaller FDRs, and high RPM/low torque motors like numerically larger FDRs. The motor I'm currently running was fast at a 3.2 FDR, but awesome at 3.6. For safety, start at a large FDR 3.6 or 3.8 (i.e., a low gear ratio) and start decreasing the FDR in increments ( I use 0.2 increments, which with my spur gear and internal gear ratio is about 2 teeth on the pinion) and note both the lap times and motor temps as you do. When the lap times stop decreasing and/or the motor temps get to 160F you have reached the sweet spot. Now go back to where the lap times have leveled off and the motor temps are the lowest and that's your FDR for that motor at that track.