Calculator
Example Data Table
| Setup | Tire Diameter | Pinion | Spur | Internal Ratio | Typical Use |
|---|---|---|---|---|---|
| Touring Carpet | 63 mm | 25T | 78T | 2.60 | Balanced indoor setup |
| Buggy Dirt | 84 mm | 22T | 81T | 2.60 | Punch and control |
| Speed Run | 68 mm | 34T | 70T | 2.10 | Higher rollout testing |
Formula Used
Tire circumference: C = π × effective tire diameter
External gear ratio: External Ratio = Spur Teeth ÷ Pinion Teeth
Final drive ratio: FDR = External Ratio × Internal Drive Ratio
Rollout: Rollout = Tire Circumference ÷ FDR
Motor RPM: RPM = KV × Voltage × Efficiency × Throttle
Estimated speed: Speed = Rollout × Motor RPM, then converted into km/h and mph.
A larger rollout means the car travels farther per motor revolution. This can increase speed, but it can also raise heat and reduce punch.
How to Use This Calculator
- Measure the tire diameter with calipers.
- Select millimeters or inches.
- Enter the pinion teeth, spur teeth, and internal drive ratio.
- Add motor KV, battery voltage, efficiency, and throttle.
- Enter a target rollout if you want gear suggestions.
- Press the calculate button.
- Read the rollout, final drive ratio, speed, and comparison table.
- Download the CSV or PDF report for your setup notes.
RC Car Rollout Guide
Why Rollout Matters
Rollout tells you how far an RC car moves for one motor revolution. It joins tire size, spur gear, pinion gear, and internal ratio into one useful number. A higher rollout means taller gearing. The car may gain speed, but the motor works harder. A lower rollout means shorter gearing. The car may launch harder and run cooler, but top speed may drop.
Better Gearing Choices
Good rollout planning helps you tune for a track, motor, and battery. Tight carpet tracks often need strong punch and controlled speed. Large asphalt tracks may need more rollout to hold speed on long straights. Dirt tracks can change during the day. Small gearing moves can keep the car stable as grip changes.
Tire Size Effect
Tire diameter is a major part of rollout. Worn tires reduce circumference. Ballooning or tire growth can increase it at speed. Two cars with the same pinion and spur can feel different when tire sizes differ. That is why measuring tire diameter is better than guessing from the label.
Motor And Heat
Taller gearing can raise motor temperature. It can also increase load on the electronic speed control. Use rollout with temperature checks after each run. If the motor is hot, reduce pinion size, increase spur size, or lower timing. If the car is cool and slow, add rollout carefully.
Using The Results
This calculator shows rollout, final drive ratio, wheel rpm, and estimated speed. It also suggests pinion and spur values for a target rollout. Treat speed as an estimate. Real speed changes with drag, battery sag, tire slip, timing, and surface grip.
Setup Workflow
Start with a safe baseline from your kit manual. Enter the measured tire diameter. Add your spur, pinion, and internal ratio. Review the rollout. Then test one change at a time. Record lap times and motor temperature. Use the comparison notes to decide whether the change helped. A consistent setup log makes future gearing choices faster and safer. Keep notes beside each setup sheet. Include weather, tire sauce, battery type, and track grip. These details explain why one rollout feels perfect on one day and weak on another day.
FAQs
What is RC car rollout?
Rollout is the distance your RC car travels for one motor revolution. It combines tire circumference, spur gear, pinion gear, and internal drive ratio into one setup number.
Is higher rollout always better?
No. Higher rollout can increase top speed, but it can reduce acceleration and raise motor temperature. Use it carefully and check heat after each full run.
How does pinion size affect rollout?
A larger pinion usually increases rollout and speed. A smaller pinion reduces rollout and improves punch. Always change pinion size in small steps.
How does spur size affect rollout?
A larger spur lowers rollout and gives shorter gearing. A smaller spur raises rollout and creates taller gearing. Both changes affect motor load.
Why enter internal drive ratio?
The internal drive ratio is part of the total gearing. Without it, the final drive ratio and rollout result will be incomplete and inaccurate.
Should I measure tire diameter?
Yes. Tire wear, inserts, compound, and ballooning can change real tire size. Measuring gives a better rollout value than using the printed tire size.
Is the speed result exact?
No. It is an estimate. Real speed depends on battery sag, drag, surface grip, tire slip, motor timing, temperature, and drivetrain condition.
How should I choose a target rollout?
Start with your kit manual, motor guide, or a proven local setup. Then adjust rollout with lap times, handling feel, and motor temperature data.