RC gear ratio: pinion, spur and theoretical top speed
Swapping a pinion by two teeth transforms an RC car — into an overheating rocket or a tireless tractor. This tool computes your setup’s final drive ratio and theoretical top speed, so you compare BEFORE pulling the motor mount apart.
Theoretical top speed
32mph
- Final drive ratio
- 13.31:1
- Wheel RPM
- 2,920RPM
The method
A brushless motor spins at a speed proportional to voltage: its kV times the battery’s volts gives the no-load RPM. A 3500KV on 3S spins around 38,850 RPM.
That speed is then geared down twice: by the pinion/spur pair (your choice) and by the chassis’ internal ratio (fixed, in the manual). Their product is the final drive ratio — motor turns per wheel turn.
Speed follows from the tire’s circumference. It is a THEORETICAL speed: no load, no friction, no air resistance — reality runs about 85% of it, and that is fine: the calculation exists to COMPARE setups, not to certify a record.
In the formula: kV and U the motor and voltage, Zs and Zp the spur and pinion teeth, i the internal ratio, D the tire diameter (mm).
ratio = (Zs / Zp) × i · speed (km/h) = kV × U / ratio × π × D × 60 / 10⁶
Good to know
- Too TALL a ratio (big pinion) overheats motor and ESC: after 5 minutes of running, if you cannot hold a finger on the motor (> 160 °F), drop one or two pinion teeth.
- Stepping up voltage (2S → 3S) is like gearing 50% taller: compensate by dropping pinion teeth, or the heat arrives within the first pack.
- Bigger tires also gear the car taller (and raise the chassis): a switch to monster tires demands the same pinion reflex.
- The displayed speed is no-load: expect ~85% in reality on flat ground, less on loose terrain. The figure’s value is comparing two setups.
- Reset gear mesh after every pinion change: a strip of paper between pinion and spur — too tight wears, too loose strips.
FAQ
- Bigger pinion or smaller spur: any difference?
-
Mathematically none: only the spur/pinion ratio matters. In practice you tune with the pinion ($5 a piece, two-minute swap) and keep the spur for big changes — it is also the part that absorbs impacts and wears fastest.
- How do I know my gearing is too tall?
-
The thermometer decides: run a representative 5 minutes, then measure motor and ESC (ideally with an IR thermometer). Above 160 °F motor temperature, drop pinion teeth. Other symptoms: sluggish corner-exit acceleration and collapsing runtime — the motor is lugging outside its efficiency band.
- Is higher kV “better”?
-
No: it is a design choice, not a performance grade. High kV spins fast with little torque per amp (on-road, light cars); low kV delivers usable torque (crawlers, 4S bashing). The kV × voltage combination must stay within what the chassis was designed for — hence calculating before buying the motor.
- Where do I find my chassis’ internal ratio?
-
In the manual’s specs (“internal drive ratio”), or on the manufacturer’s product page. Failing that, count teeth through the whole drivetrain, or measure empirically: motor turns per wheel turn with the slipper loosened. Typical values: 2.5–2.7 for 1/10 buggies, around 1.0 for direct belt on-road, 5+ for crawlers.
- Why is my GPS speed far from the calculation?
-
Three stacked reasons: a LiPo’s real voltage sags below nominal under load, the motor loses revs under aero and mechanical load, and tires deform (their diameter actually balloons at speed). A 10–20% gap is normal; beyond that, look for mechanical drag — mesh too tight, tired bearings.