RC Gear Ratio and Rollout Calculator
Enter the spur, pinion, internal ratio, tire diameter, and motor numbers to analyze a current RC car setup or solve for a target rollout or speed. The calculator compares available whole-tooth pinions instead of stopping at the algebraic answer.
Recommended pinion24T
- Ideal pinion
- 23.65
- Final drive ratio
- 7.8
- Rollout
- 25.37
- Estimated loaded motor RPM
- 22,015
- Estimated top speed
- 33.52
- Target error
- 1.5%
- Gearing change from current
- 0%
Bracketing pinions
| Pinion | FDR | Rollout | Speed | Target error | Status |
|---|---|---|---|---|---|
| 23 | 8.14 | 24.32 | 32.12 | -2.73% | Below the ideal pinion. |
| 24 | 7.8 | 25.37 | 33.52 | 1.5% | Closest available pinion. |
Estimated speed uses Kv, voltage, and the loaded RPM factor; it does not model drag, tire slip, battery sag, boost timing, or drivetrain losses beyond that factor.
Check motor and ESC temperatures after a gearing change.
How to use this calculator
- Choose whether to analyze the installed gearing or solve for target rollout or speed.
- Enter spur teeth, current pinion teeth, internal ratio, and driven tire diameter.
- Enter motor Kv, battery voltage, and the loaded RPM factor for speed estimates.
- Set the smallest and largest pinions you actually have available.
- Read the recommended pinion, then compare the nearby pinions and target error.
How the gearing numbers are calculated
RC touring cars, buggies, pan cars, and drag cars usually describe gearing with final drive ratio and rollout. Final drive ratio combines the external spur and pinion pair with the car's internal transmission ratio:
FDR = (spur teeth ÷ pinion teeth) × internal ratio
Rollout is the distance the car travels for one motor revolution. It uses the driven tire circumference divided by final drive ratio:
rollout = pi × tire diameter ÷ FDR
A larger pinion lowers final drive ratio and increases rollout. That is taller gearing: more distance per motor revolution, more theoretical speed, and usually more motor load. A smaller pinion raises final drive ratio and lowers rollout, which is shorter gearing.
Solving backward for a pinion
When you choose target rollout, the calculator rearranges the rollout formula to solve the ideal pinion tooth count:
ideal pinion = target rollout × spur teeth × internal ratio ÷ (pi × tire diameter)
When you choose target speed, the tool first estimates loaded motor RPM from motor Kv, voltage, and the loaded RPM factor:
loaded RPM = Kv × voltage × loaded RPM factor
Estimated speed is then:
speed = loaded RPM × rollout × 60 ÷ 1,000,000
Solving that equation for pinion teeth gives:
ideal pinion = target speed × 1,000,000 × spur teeth × internal ratio ÷ (60 × pi × tire diameter × Kv × voltage × loaded RPM factor)
Pinions are sold as whole teeth, so the calculator enumerates every integer pinion from the available minimum to the available maximum. It recommends the pinion with the smallest absolute error from the selected target and shows the nearby bracketing pinions with their final drive ratio, rollout, speed, and signed error. If the ideal value lands exactly halfway between two teeth, both are marked equally close.
What moves the result most
Tire diameter matters because it changes circumference directly. Foam tires lose rollout as they wear; rubber tires may grow at speed. Internal ratio is just as important: entering 1 for a direct-drive car is correct, but a shaft or belt car needs the published transmission ratio for its chassis. Motor Kv, voltage, and loaded RPM factor affect the speed estimate and any speed-based pinion solution, but they do not change the rollout-only pinion recommendation.
What this calculator leaves out
The speed estimate is not a dyno or radar reading. It leaves out aerodynamic drag, tire slip, tire growth, battery sag, drivetrain loss beyond the entered loaded RPM factor, motor timing, ESC boost or turbo, and track surface. It also cannot know whether a large pinion will clear the motor mount, mesh correctly, or keep motor and ESC temperatures within the manufacturer's limits.
Worked example
Suppose an on-road car uses a 72-tooth spur, a 24-tooth pinion, a 2.60:1 internal ratio, and 63 mm driven tires. The final drive ratio is (72 ÷ 24) × 2.60 = 7.80. Tire circumference is about 197.92 mm, so the current rollout is 197.92 ÷ 7.80 = 25.37 mm per motor revolution.
With a 3500 Kv motor on 7.4 V and an 85% loaded RPM factor, loaded motor speed is about 22,015 RPM. That makes the estimated loaded top speed about 33.51 km/h. If the target rollout is 25.0 mm per motor revolution and available pinions are 15T through 40T, the ideal pinion is 23.64T and the closest whole-tooth recommendation is 24T, about 1.5% above the target rollout.
Common questions
What is final drive ratio in an RC car?
Final drive ratio is the total gear reduction from the motor to the driven wheels. For a common spur and pinion setup, it is spur teeth divided by pinion teeth, then multiplied by the chassis internal ratio. A higher final drive ratio is shorter gearing, while a lower final drive ratio is taller gearing.
What does rollout mean in RC racing?
Rollout is the distance the car travels for each motor revolution. It is tire circumference divided by final drive ratio. Rollout is useful when tire diameter changes, because the same gears can produce different rollout on worn foam tires or larger rubber tires.
Does a larger pinion make an RC car faster?
A larger pinion usually raises theoretical speed because it lowers final drive ratio and increases rollout. The car still has to pull that gearing, so measured speed can fall if the motor bogs, the battery sags, or temperatures climb. Check motor and ESC temperature after moving to a larger pinion.
Can I solve for a pinion from a class rollout limit?
Yes. Choose target rollout mode and enter the class limit as the target rollout. The calculator returns the closest whole-tooth pinion in your available range and shows whether the nearby choices are above or below the target.
How should I choose the loaded RPM percentage?
Use a lower value when the setup is heavily loaded, the pack sags, or the motor is not reaching no-load RPM on track. Values around 80% to 90% are common planning estimates, but telemetry, gearing logs, or measured speed are better for tuning a specific car.
Why is measured speed lower than the calculated speed?
The calculated speed assumes the motor reaches the entered loaded RPM and the tire turns without slip. Real cars lose speed to aerodynamic drag, rolling resistance, drivetrain friction, battery sag, tire deformation, and limited power. ESC timing and boost can also make the real result differ from a simple Kv estimate.