RC Car Gear Ratio Calculator

Calculate final drive ratio and theoretical top speed for RC cars based on pinion, spur, diff ratio, and motor specs.

Inputs

Pinion Teeth

Spur Gear Teeth

Differential Ratio

Motor Kv Rating

Battery Voltage

Tab to move between fields

Results

Final Drive Ratio

Theoretical Speed (mph)

26.4

Motor Max RPM22,200

Torque Multiplier10x

How to Use This Calculator

Enter Pinion Teeth, Spur Gear Teeth, and Differential Ratio.
Set Motor Kv Rating and Battery Voltage.
Review Final Drive Ratio and Theoretical Speed (mph).
Use Motor Max RPM and Torque Multiplier (x) to inform your decision.
Use the chart to visualize the results and explore different scenarios by adjusting inputs.

How the result changes with Pinion Teeth

Pinion Teeth	Final Drive Ratio	Theoretical Speed (mph)
9.5	21.05	12.6
21	9.52	27.8
34	5.88	44.9
46	4.35	60.7

What each input means

Pinion Teeth: Number of teeth on the motor pinion gear
Spur Gear Teeth: Number of teeth on the spur (main) gear
Differential Ratio: Internal gear ratio of the differential
Motor Kv Rating: Motor RPM per volt (higher Kv = faster, lower torque)
Battery Voltage: Battery nominal voltage (e.g., 7.4V for 2S LiPo)

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
Pinion Teeth = 20, Spur Gear Teeth = 80, Differential Ratio = 2.5, Motor Kv Rating = 3000 = 5 input(s) provided
Calculate Final Drive Ratio
Final Drive Ratio
10 = 10
Calculate Theoretical Speed
Theoretical Speed
26.4 = 26.4
Calculate Motor Max RPM
Motor Max RPM
22200 = 22200
Calculate Torque Multiplier
Torque Multiplier
10 = 10

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