What one pinion tooth changes

One tooth is a percentage change

A change from 13T to 14T adds about 7.7% to theoretical wheel speed at a fixed motor RPM. A change from 20T to 21T adds 5%. The extra tooth is the same size of step in the parts drawer, but it is not the same change in the gearing.

The useful comparison is new pinion ÷ old pinion. Subtract one and multiply by 100 for the percentage change. This works only when the spur, internal reduction, tires and motor RPM stay the same.

Work through a Rival MT10 example

The Rival MT10 V2 20518 reference uses 12T/54T and a 2.85 internal reduction. Final drive is 54 ÷ 12 × 2.85, about 12.83:1. A hypothetical 13T pinion gives 54 ÷ 13 × 2.85, about 11.84:1.

With a 3300 KV motor, nominal 3S voltage and an estimated 120 mm tire diameter, those setups calculate to about 40 mph and 43 mph. The tire figure is an estimate, not a manufacturer-confirmed dimension; measure your fitted tires before relying on the absolute speeds. The relative increase is still 13 ÷ 12, about 8.3%, when every other input stays fixed.

Check the exact-model manual (opens in a new tab) for the fitted pinion and mesh instructions. The 13T case is a calculation example, not a documented recommendation. Confirm mounting range, gear pitch, shaft bore and operating limits before changing the hardware.

The speed percentage and torque percentage differ

At the same motor torque, ideal wheel torque is proportional to final drive. Going from 12T to 13T gives 12 ÷ 13 of the previous multiplication, about 7.7% less. Speed rises by about 8.3%, while torque multiplication falls by about 7.7%, because the two percentages start from different baselines.

That is not a direct acceleration prediction. Grip, motor torque, speed control limits and battery behavior all matter. A taller setup can make the motor draw differently and settle at a lower RPM under load, so some of the calculated speed gain can disappear.

Tires change the effective gearing

Road speed also changes in proportion to tire diameter. Moving from the estimated 120 mm baseline to a hypothetical 130 mm tire increases distance traveled per wheel turn by about 8.3%. Keeping the same gears would therefore make the rollout taller by the same proportion as the 12T-to-13T pinion change.

To preserve the original rollout with those larger tires, the arithmetic asks for 12 × 120 ÷ 130, or about 11.1 teeth. A pinion must have a whole tooth count, and mounting range and manufacturer guidance still apply. Use the tire and gearing calculator to compare feasible choices rather than rounding straight into a purchase.

Make the test tell you something

1. Save a baseline. Record pinion, spur, internal reduction, tire diameter, battery and voltage basis in the speed calculator.
2. Change one input. Keep the tire and battery assumptions identical so the calculated difference comes from the pinion.
3. Check the physical combination. Confirm pitch, shaft bore, clearance and mesh using the manufacturer’s instructions.
4. Compare under similar conditions. Use the same surface and comparable battery state. Record temperature with an appropriate instrument and follow the equipment limits.
5. Keep calculation and measurement separate. A GPS result answers what happened in that run; a no-load result explains the gearing assumptions.

If the taller setup gives little extra measured speed while temperatures increase, another tooth is not automatically the answer. Inspect drag, gearing suitability, battery behavior and the available run-up before changing again.

Reference sheets

• Team Associated Rival MT10 V2. Manufacturer reference (opens in a new tab).

Try it with your model

Compare a pinion change

Loading a vehicle changes the working setup shared by the car calculators. Save your current setup first if you want to keep it.

Sources and corrections

Reference figures above come from the linked vehicle sheets and their manufacturer documents. No hands-on fitment or speed test is claimed. For the evidence standard, see Methodology. Send a correction with the exact model and source to hello@rctunelab.com.