RC Car Gear Ratio & Top Speed Calculator

This RC car gear ratio calculator turns your motor KV, battery, pinion, spur, internal ratio and tire size into final drive ratio, rollout, wheel rpm and top speed in mph and km/h. Results are theoretical no-load maximums, not real-world speeds.

How to use the RC car gear ratio calculator

1. Pick a preset, or leave it on “None”. The 1/10 Traxxas Slash 4X4 VXL preset fills in the stock values from its owner’s manual. The 1/8 preset is a blank template, because no manual could be sourced for it.
2. Enter the motor KV from the motor label or manual.
3. Enter the LiPo cell count (2S, 3S and so on). If you know the actual pack voltage, type it into the override field instead; the override replaces the cell count.
4. Enter the pinion and spur tooth counts, and the internal ratio (transmission and differential) from your manual.
5. Measure the mounted tire’s outer diameter and enter it in millimetres or inches.
6. Read the results. They update as you type, and “Show the maths” lists every substitution so you can check it by hand.
7. Optional: enter a start and end pinion (up to 10 rows) to see FDR, rollout and theoretical speed for each pinion with everything else held fixed.

The calculator only works from the numbers you enter. It does not suggest a pinion, a ratio or a “safe” speed.

How the RC car speed calculator works (the formulas)

Every step is plain arithmetic, so you can reproduce any result with a pocket calculator:

1. Battery voltage: V = cells × 3.7 (nominal LiPo). A typed voltage overrides the cell count.
2. Final drive ratio: FDR = (spur ÷ pinion) × internal ratio
3. No-load motor rpm: motor_rpm = KV × V
4. Wheel rpm: wheel_rpm = motor_rpm ÷ FDR
5. Rollout: rollout_mm = (π × tire_OD_mm) ÷ FDR, and inches = mm ÷ 25.4
6. Theoretical top speed: speed_m_per_min = wheel_rpm × π × tire_OD_mm ÷ 1000, then kph = speed_m_per_min × 60 ÷ 1000 and mph = kph ÷ 1.609344

KV is defined as no-load rpm per volt, so every speed here is a theoretical no-load maximum. The 3.7 V per cell figure is a nominal assumption: a LiPo cell runs from about 4.2 V fully charged down to roughly 2.7–3.0 V when discharged, and it drops further under load.

Worked example (an arithmetic test case, not a specific car): 3000 KV motor, 2S LiPo, 48-tooth spur, 16-tooth pinion, 2.5 internal ratio and a 64 mm tire.

Final drive ratio, rollout and crawlers: what the numbers mean

Final drive ratio is how many times the motor turns for one turn of the wheels. The spur-to-pinion ratio is the spur’s tooth count divided by the pinion’s. That figure is multiplied by the car’s internal ratio (transmission and differential) to give the FDR. Traxxas uses the same method in the Slash 4X4 manual’s gearing chart: spur teeth ÷ pinion teeth × 2.85.

Rollout is how far the car moves for one motor revolution. It is the tire circumference divided by the FDR, so it combines gearing and tire size into one number.

If you came here looking for an RC car gear calculator or a pinion size chart, here is what the teeth do mathematically. A larger pinion or a smaller spur lowers the FDR and raises rollout and theoretical speed. A smaller pinion or a larger spur does the opposite. Pinion is the denominator in the ratio, so even when the pinion has more teeth than the spur, the formula still works and the FDR simply drops below the internal ratio. Use the pinion sweep to see a chart for your own setup. Your manufacturer’s manual lists the pinion and spur combinations it supports.

The tool also works as an RC crawler gear ratio calculator. Crawlers use the same formulas. Enter the overall internal ratio from your crawler’s manual, multiplying the stage ratios together if the manual lists the transmission, transfer case and axle or portal ratios separately. For crawlers, rollout and wheel rpm are often more useful than the top-speed figure.

Why real speed differs from this number

• Drivetrain losses in the gears, bearings and differentials
• Battery voltage sag under load and as the pack discharges
• Aerodynamic drag and rolling resistance
• Tire growth (ballooning) at speed, which changes the effective diameter
• ESC and motor limits, including any timing or speed settings
• Traction, surface and the space available to reach full speed

None of these factors is modeled, so treat the result as a theoretical ceiling for comparing setups on paper, not as a measured speed. It does not confirm any manufacturer’s speed claim.

Methodology & sources

Everything is calculated in your browser from the values you enter; nothing is stored or sent. Constants are exact unit definitions, the 3.7 V cell voltage is a labelled nominal assumption, and the only preset values come from the cited Traxxas manual. The 1/8 preset is a blank template because no manual could be sourced.

• Wikipedia: Inch (1 inch = 25.4 mm exactly)
• Wikipedia: Mile (1 mile = 1.609344 km exactly)
• Wikipedia: Lithium polymer battery (nominal cell voltage 3.6–3.7 V; real voltage varies with charge)
• Wikipedia: Motor constants (Kv = unloaded rpm per volt)
• Wikipedia: Gear train (gear ratio from tooth counts)
• Traxxas Slash 4X4 owner’s manual 68386-4 (preset: 3500 kV motor, 13/54 stock gearing, 2.85 internal ratio, 3S LiPo 11.1 V nominal; FDR formula)

These figures are theoretical no-load maximums for information only and are not a recommendation for gearing or speed. Real speeds differ, and RC cars, motors and LiPo batteries can cause injury or fire if misused, so follow your manufacturer’s manuals and limits and run only in a safe area away from people, pets and traffic.

Last updated: October 2026

FAQ

How fast is my RC car?

Enter your motor KV, battery, pinion, spur, internal ratio and tire diameter, and the calculator returns a theoretical no-load maximum speed in mph and km/h. Real speed differs because the model leaves out drivetrain loss, battery sag, drag, traction and tire growth.

How do I calculate FDR?

Final drive ratio is FDR = (spur ÷ pinion) × internal ratio. For example, a 54-tooth spur, 13-tooth pinion and 2.85 internal ratio give 54 ÷ 13 × 2.85 = 11.84.

Does this tell me what pinion to use?

No. The calculator only computes from the values you enter and does not recommend any pinion, spur or ratio. Follow your manufacturer’s manual for gearing limits.

Why does the calculator use 3.7 V per cell?

3.7 V is the nominal voltage of a LiPo cell, so 2S is 7.4 V and 3S is 11.1 V. Real cell voltage varies with charge state and load, so you can type a measured voltage into the override field instead.

Can I use it as an RC crawler gear ratio calculator?

Yes. The same formulas apply to crawlers; enter your crawler’s overall internal ratio from its manual, multiplying the stage ratios together if the manual lists them separately. The rollout figure shows how far the truck moves per motor revolution.

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