RC Gear Ratio Calculator
Use this calculator to find your external gear ratio, final drive ratio (FDR), rollout, and estimated top speed.
What Is an RC Gear Ratio?
Your RC car’s gear ratio controls how motor RPM is converted into wheel speed and torque. In simple terms, it determines whether your setup favors quick acceleration and cool running, or higher top-end speed with more heat and stress.
There are two important values:
- External Gear Ratio: Spur teeth divided by pinion teeth.
- Final Drive Ratio (FDR): External ratio multiplied by the internal transmission ratio.
A higher FDR generally means more torque and lower top speed. A lower FDR generally means higher top speed and lower torque.
Formulas Used in This Calculator
- External Gear Ratio = Spur Teeth ÷ Pinion Teeth
- Final Drive Ratio (FDR) = External Gear Ratio × Internal Ratio
- Tire Circumference (mm) = π × Tire Diameter
- Rollout (mm per motor revolution) = Tire Circumference ÷ FDR
- No-Load Motor RPM = Motor KV × Battery Voltage
- Wheel RPM = Motor RPM ÷ FDR
The speed estimate also applies your efficiency percentage, because real cars lose power through drivetrain friction, tire deformation, and load.
How to Use This RC Gear Ratio Calculator
Step 1: Enter spur and pinion gear teeth
These are the gears you physically install. If you increase pinion size or reduce spur size, you are gearing taller (lower FDR).
Step 2: Enter internal ratio
This is specific to your model (gearbox + diff relationship). It is fixed unless you change internal gears. Most manufacturers publish this in the manual.
Step 3: Enter tire diameter
Taller tires effectively gear the car up, just like using a larger pinion. Even a few millimeters can noticeably change performance.
Step 4: Add motor KV and voltage (optional, but recommended)
With KV and battery voltage, the tool can estimate wheel RPM and top speed in km/h and mph.
How to Pick the Best Gearing
For bashing and off-road
- Use a slightly higher FDR for better punch and cooler temps.
- Prioritize motor safety and drivability over peak speed.
- Check motor temps after each run.
For on-road speed and speed runs
- Use a lower FDR to increase top-end.
- Make small changes: 1 tooth on pinion can be significant.
- Monitor motor and ESC temperatures closely.
For racing
- Tune to track size and grip level.
- Short technical track: gear shorter for acceleration out of corners.
- Long straightaway track: gear taller for maximum straight speed.
Common RC Gearing Mistakes
- Ignoring temperature: Fast gearing that overheats your motor is not a win.
- Using only top speed as a target: Lap time often depends more on acceleration and consistency.
- Forgetting tire changes: New tire size changes your effective gearing immediately.
- Skipping mesh checks: Bad gear mesh can destroy spur/pinion gears quickly.
Quick FAQ
Is a higher gear ratio faster?
It depends on what ratio you mean. In RC, a lower final drive ratio usually gives higher top speed, while a higher FDR usually gives stronger acceleration and lower top speed.
What is a safe first change?
Try changing pinion by 1 tooth at a time, then test motor temperature. Small increments are safer and easier to evaluate.
Can this calculator replace real-world testing?
No. It gives a strong starting point, but real-world conditions (surface, wind, traction, battery sag, and vehicle weight) still matter.
Final Thoughts
A good RC gear ratio setup is about balance. This calculator helps you make informed gearing changes using real numbers instead of guesswork. Start conservatively, test often, and keep your electronics in a safe temperature range.