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Work out overall reduction from spur and pinion tooth counts and an optional internal ratio from your transmission or center diff. Optionally add motor RPM and tire outer diameter in millimeters for a simple rollout speed estimate in mph and km/h.
Last updated: April 17, 2026
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Large gear on the layshaft or spur mount.
Small gear on the motor shaft.
Multiply spur/pinion by this when your vehicle has a fixed center diff or gearbox ratio (use 1 if not applicable).
Measured outer diameter; both RPM and tire are required for mph/kph.
Overall ratio (motor revs : 1 wheel rev)
19.046 : 1
Wheel RPM
1,312.6 rpm
Est. top speed
16.6 mph (26.72 km/h)
Notes:
Base factor
spur ÷ pinion
Optional
× manufacturer value
Optional
mph / km/h
87 / 13 spur–pinion, internal 2.846, 25k motor RPM, 108 mm tire — overall ratio about:
19.05 : 1
~16.6 mph (26.72 km/h) rollout estimate
Tooth-driven stages multiply. The tool divides spur teeth by pinion teeth to capture how much the motor must turn relative to the spur, then multiplies by your internal ratio when that stage exists. Wheel RPM divides motor RPM by that overall factor. For speed, it converts tire diameter from millimeters to feet per revolution and multiplies by wheel RPM with standard mile-per-hour unit algebra.
overall = (spur / pinion) × internalwheelRPM = motorRPM / overallmph ≈ wheelRPM × (π × tireDiaIn / 12) × (60 / 5280)~16.6 mph
Change pinion or tire diameter to compare alternatives before you buy parts.
Share it with RC drivers comparing pinion and spur changes.
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