What does the gear ratio mean for my RC car?

A higher gear ratio (more spur teeth or fewer pinion teeth) increases torque and acceleration but reduces top speed, while a lower ratio increases speed but reduces torque.

How do I measure tire diameter accurately?

Measure the outer diameter of the tire when it is mounted and inflated to its normal operating pressure, ideally under the weight of the vehicle.

Why is efficiency important in the speed calculation?

Efficiency accounts for energy losses in the motor and drivetrain; using a realistic value (like 85%) gives a more accurate estimate of real-world speed.

Can I use this calculator for brushless and brushed motors?

Yes, but note that brushed motors typically have lower efficiency (around 70-80%) and their kV rating may be less accurate under load.

RC Gear Ratio Calculator (Spur/Pinion, Rollout, Speed)

Enter the number of teeth on your spur gear and pinion gear, along with your transmission's internal gear ratio, tire diameter, motor kV, battery voltage, and efficiency to compute your RC car's gear ratio, rollout, and speed.

How It Is Calculated

The RC Gear Ratio Calculator determines the effective gear ratio, rollout, and expected speed of a radio-controlled vehicle based on the motor, battery, and drivetrain parameters. The calculations follow these steps:

Gear Ratio

The gear ratio (G) is the ratio of spur gear teeth (S) to pinion gear teeth (P):

G = S / P

Overall Ratio

The overall ratio (O) includes the internal gearbox ratio (I):

O = G × I

Rollout

Rollout (R) is the distance the vehicle moves per motor revolution, calculated from tire circumference divided by the overall ratio:

R = (π × D) / O

where D is the tire diameter in inches.

Motor RPM

Motor RPM (M) is the product of motor kV (K), battery voltage (V), and efficiency (E as a decimal):

M = K × V × (E / 100)

Speed

Speed (S) in miles per hour is derived from motor RPM and rollout:

S = (M × R × 60) / 63360

All intermediate values use full precision; final outputs are rounded as shown.

Assumptions

Motor kV is the no-load RPM per volt.
Efficiency accounts for motor and drivetrain losses; 85% is typical for brushless systems.
Internal gear ratio is 1 if no additional gearbox is present.
Tire diameter is the outer diameter under load (approximate).
Battery voltage is nominal (e.g., 7.4V for 2S LiPo).

Worked Example

Using the default inputs:

Spur gear teeth (S) = 100 teeth
Pinion gear teeth (P) = 20 teeth
Internal gear ratio (I) = 1 :1
Tire diameter (D) = 4.0 in
Motor kV (K) = 3000 RPM/V
Battery voltage (V) = 7.4 V
Efficiency (E) = 85%

Step 1: Gear ratio G = 100 / 20 = 5 :1

Step 2: Overall ratio O = 5 × 1 = 5 :1

Step 3: Rollout R = (π × 4.0) / 5 = 12.56637 / 5 = 2.51327 in (rounded to 2.51 in)

Step 4: Motor RPM M = 3000 × 7.4 × (85/100) = 3000 × 7.4 × 0.85 = 18870 RPM

Step 5: Speed S = (18870 × 2.51327 × 60) / 63360 = (18870 × 150.7962) / 63360 = 2845000 / 63360 ≈ 44.9 mph

Results: Gear ratio = 5 :1, Overall ratio = 5 :1, Rollout = 2.51 in, Speed = 44.9 mph, Motor RPM = 18870 RPM.

Practical Tips

Use a motor torque calculator to check if your motor can handle the load at the chosen gear ratio.
Smaller pinion gears increase torque but reduce top speed; larger pinions do the opposite.
Always verify tire diameter under load, as it can differ from the nominal size.
For multi-speed gearboxes, enter the internal ratio for the gear you plan to use.
Efficiency varies with temperature and load; test your setup to fine-tune.

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Last reviewed: 2026-10-17. Results are estimates for planning. Check them before you rely on them for design, purchase or safety decisions.

RC gear ratio calculator