RC gear ratio, FDR and rollout calculator

How to use it

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What changing a tooth does

A bigger pinion or a smaller spur lowers the FDR and raises rollout. The car gets more top speed, less punch off the corner, and the motor runs hotter.

A smaller pinion or a bigger spur does the opposite: more punch, less top speed and a cooler motor. Change one tooth at a time and check motor temperature after each run.

Pan car rollout vs geared cars

Pan cars are direct drive. The spur sits on the rear axle, so the internal ratio is 1 and FDR is just spur divided by pinion. Pan car racers often talk in rollout instead of FDR, because foam tires shrink as they wear and rollout includes tire size.

Buggies, trucks and short course cars run a transmission, so the internal ratio matters. Two cars with the same spur and pinion can end up with very different FDR if their transmissions differ.

Common questions

Where do I find the internal ratio?

Look in your car's manual or on the maker's spec page. Team Associated lists 2.6:1 for the RC10B6.1D, and Traxxas lists 2.72 for the Slash 2WD. Direct-drive pan cars use 1.

Does tire diameter change FDR?

No. FDR only counts gears. Tire diameter changes rollout, which is why two cars with the same FDR can run different speeds.

Should I enter diameter or radius?

Enter diameter. If you measured radius, double it. Measure straight across the center of the tire with calipers.

What rollout should I run?

It depends on the motor, the track size and your class rules. Some classes limit gearing or rollout. Start from your car's setup sheet, then change one tooth at a time.

Why does my motor run hotter after a gear change?

A lower FDR, from a bigger pinion or a smaller spur, makes the motor work harder for each turn of the tire. Check the temperature after a few laps any time you gear up.