Tire Swap Speed & Gearing Compensator

This RC car speed calculator shows how swapping to a bigger or smaller tire changes your car’s theoretical no-load top speed. Enter motor KV, battery cells, spur, pinion, internal ratio and both tire diameters to get old and new speed, the percent change and rollout. All results are theoretical estimates, not real-world or safe speeds.

How to use the tire swap speed calculator

1. Enter your motor’s KV rating (rpm per volt) from the motor’s spec sheet.
2. Enter the number of LiPo cells (for example, 2 for a 2S pack). The voltage per cell defaults to the nominal 3.7 V; change it if your pack’s nominal voltage differs.
3. Enter spur and pinion tooth counts, then the internal ratio (the transmission/differential ratio after the spur) from your vehicle manual.
4. Measure or look up the diameter of your current tire and the new tire in millimeters.
5. Pick km/h or mph. Results update as you type; use “Copy result” to paste a plain-text summary elsewhere.

The calculator does not look up KV, internal ratio or tire sizes for you. Use your own manufacturer documentation or measure the tire with calipers.

How the speed formula works

All values are theoretical and no-load. The calculator follows these steps:

1. Pack voltage = cells × voltage per cell (default 3.7 V, a nominal LiPo assumption).
2. No-load motor rpm = KV × pack voltage.
3. Total gear ratio = (spur ÷ pinion) × internal ratio.
4. Tire circumference (mm) = π × diameter (mm).
5. Speed (km/h) = rpm × circumference ÷ (total ratio × 1,000,000) × 60.
6. Speed (mph) = km/h ÷ 1.609344.
7. % change = (new speed ÷ old speed − 1) × 100, which equals (new diameter ÷ old diameter − 1) × 100 when everything else is unchanged.
8. Rollout (mm per motor revolution) = circumference ÷ total ratio.

Worked example (illustrative numbers, not a real car): KV 3000, 2 cells (7.4 V), spur 48, pinion 16, internal ratio 2.5, old tire 64 mm, new tire 68 mm. Motor rpm is 3000 × 7.4 = 22,200. Total ratio is 48 ÷ 16 × 2.5 = 7.5, so the wheels turn at 2,960 rpm.

Note that 68 ÷ 64 = 1.0625, the same +6.25% as the speed change. The percent change depends only on the tire diameter ratio; motor, battery and gearing set the absolute speeds.

Why tire diameter works like a gear ratio in this RC car gearing calculator

In this RC car gearing calculator, rollout connects tire diameter to gearing: tire circumference divided by total gear ratio gives the distance traveled per motor revolution. A larger tire increases rollout, as does a larger pinion when all other inputs remain unchanged. That is why the tool treats tire diameter as a gearing variable and reports rollout for both the old and the new tire.

This tool is not an RC car pinion size chart. It deliberately does not output a pinion size or a “correct” gearing. Gearing limits depend on your motor, ESC, cooling and vehicle, so check those manufacturers’ documentation before changing gears or tires.

What this calculator does not tell you

Results are no-load only. The model ignores load, aerodynamic drag, traction, tire growth or squish at speed, battery voltage sag and temperature. It gives no motor, ESC or safe-speed advice, does not verify any product’s advertised speed, and does not recommend a pinion.

Methodology & sources

Every number on this page is either entered by you, produced by the formulas above, or a labelled illustrative example. No third-party product data, prices or benchmarks are used, and nothing you type is stored or sent anywhere; the calculation runs in your browser.

• Wikipedia: Motor constants: KV as no-load rpm per volt.
• Traxxas owner’s manual 68386-4: KV definition, 2S = 7.4 V and 3S = 11.1 V nominal, final gear ratio structure.
• Wikipedia: Lithium polymer battery: nominal cell voltage of 3.6–3.7 V.
• Wikipedia: Gear train: gear ratio as a ratio of tooth counts.
• Wikipedia: Mile: 1 mile = 1.609344 km exactly.

Last updated: October 2026

FAQ

Do bigger tires make my RC car faster?

In theory, yes. With motor, battery and gearing unchanged, theoretical no-load speed rises by the same percentage as the tire diameter, so a 64 mm to 68 mm swap adds 6.25%. Real-world speed also depends on load, traction and battery sag, which this calculator does not model.

How is RC car speed calculated?

No-load motor rpm is KV × pack voltage. Divide that by the total gear ratio (spur ÷ pinion × internal ratio) to get wheel rpm, then multiply by tire circumference (π × diameter) and convert to km/h or mph.

Why does the calculator assume 3.7 V per cell?

3.7 V is the commonly stated nominal voltage of a LiPo cell, so 2S is 7.4 V and 3S is 11.1 V. The value is editable if your pack uses a different nominal voltage.

What is rollout?

Rollout is the distance the car travels for one motor revolution: tire circumference divided by total gear ratio. A larger tire increases rollout just like a larger pinion does.

Does this calculator recommend a pinion or tell me if a swap is safe?

No. It shows only the theoretical, no-load speed change. Check your motor, ESC and vehicle manufacturer’s documentation for gearing and temperature limits.

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This calculator gives a theoretical, no-load estimate only and is not safety advice. Real speed and component load depend on conditions this tool does not model. Always follow your motor, ESC, battery, and vehicle manufacturer’s limits and operate RC vehicles safely and legally.