What is gear ratio in an RC car?

The gear ratio is the number of times the motor shaft turns for every one turn of the drive axle. It equals spur-gear teeth divided by pinion-gear teeth. A 72T spur and 25T pinion gives 72 ÷ 25 = 2.88:1, meaning the motor spins 2.88 times per one wheel rotation. A higher ratio increases torque and traction but lowers top speed; a lower ratio does the opposite.

How is top speed calculated from gear ratio?

No-load motor RPM = Kv × battery voltage. Drive-shaft RPM = motor RPM ÷ gear ratio. Top speed (m/s) = drive-shaft RPM × wheel circumference (π × diameter in metres) ÷ 60. The result is multiplied by 3.6 for km/h. Real-world speed is typically 15–25% lower due to drivetrain friction, motor load and tire slip.

What gear ratio should I use for racing on tarmac?

Tarmac racing generally uses ratios in the 2.5:1–3.5:1 range for 1/10 touring cars. Lower ratios (bigger pinion or smaller spur) maximise top speed but run hotter; higher ratios improve punch out of slow corners. Start at the manufacturer-recommended ratio and tune one tooth at a time based on motor temperature after a run.

What is motor Kv and how does it affect gearing?

Kv is the motor's velocity constant — the RPM it spins per volt with no load attached. A 3500 Kv motor on a 7.4 V (2S LiPo) pack spins at approximately 25,900 RPM unloaded. Higher Kv motors spin faster and typically need a lower gear ratio (smaller pinion) to keep temperatures safe. Lower Kv motors have more torque and can use a higher ratio.

Can I mix different pitch gears?

No. Pinion and spur gears must share the same pitch (module). The two most common RC pitches are 48-pitch (48P) for 1/10 scale and 32-pitch (32P) for 1/8 scale. Mixing pitches causes stripped teeth and poor mesh. Always confirm pitch before buying replacement gears.

How do I stop my motor from overheating with gearing changes?

After each change run the car for 3–5 minutes at full throttle, then immediately touch the motor. If it is too hot to hold for two seconds, add one tooth to the spur gear (raises ratio, reduces speed but lowers temperature). If it is cool, try removing a tooth from the spur or adding one to the pinion to squeeze out more speed. Most brushless motors are safe below 80°C (176°F).

RC Gear Ratio Calculator

The RC gear ratio is the single most impactful tuning variable on any remote-control car. Change it and you change top speed, acceleration, motor temperature and even battery run time — all without touching the electronics. This calculator gives you the exact ratio, drive-shaft RPM and estimated top speed from your pinion and spur tooth counts, plus the ability to work backwards from a target ratio or a desired speed to find the pinion you need to buy.

How it works

The fundamental formula is beautifully simple:

Gear Ratio (GR) = Spur Teeth ÷ Pinion Teeth

A 72-tooth spur paired with a 25-tooth pinion produces GR = 72 ÷ 25 = 2.88:1. The motor shaft turns 2.88 times for every single revolution of the spur gear (and, after any internal drive-train reduction, the drive axle).

To convert that into speed the calculator chains three more steps:

No-load motor RPM = Kv × battery voltage. A 3,500 Kv motor on a 7.4 V 2S LiPo pack produces roughly 25,900 RPM at the pinion.
Drive-shaft RPM = motor RPM ÷ gear ratio. At 2.88:1 that is about 8,993 RPM at the spur.
Pitch speed = drive-shaft RPM × wheel circumference (π × diameter in metres) ÷ 60 seconds.

For a 60 mm-diameter tyre the circumference is π × 0.060 ≈ 0.1885 m, giving a theoretical top speed of 8,993 × 0.1885 / 60 ≈ 28.2 m/s = 101 km/h. Real-world speed is 15–25% lower because of motor load, drivetrain losses and tyre flex.

The solve-for modes rearrange these formulas algebraically:

Solve for spur → Spur = Pinion × Target ratio
Solve for pinion → Pinion = Spur ÷ Target ratio
Solve for pinion from speed → Pinion = (V_target × 60 × Spur) ÷ (Kv × Voltage × π × Diameter)

Worked example

Suppose you are building a 1/10 touring car with a 3,500 Kv motor, 7.4 V 2S pack and 60 mm tyres. You start with a 25T pinion and 72T spur:

Metric	Value
Gear ratio	2.88:1
Motor no-load RPM	25,900
Drive-shaft RPM	8,993
Estimated top speed	~101 km/h (63 mph)
Torque multiplier	2.88×

The motor runs hot after a few laps. You switch to a 23T pinion (ratio = 72 ÷ 23 = 3.13:1), dropping estimated speed to ~93 km/h but raising the torque multiplier and cutting motor temperature noticeably. One tooth change is all it takes.

Now suppose you want to hit exactly 80 km/h with that same motor on the same 72T spur. Switch the calculator to “Solve for pinion from speed” and it tells you: Pinion = (80/3.6 × 60 × 72) ÷ (3,500 × 7.4 × π × 0.060) ≈ 18T — so you need an 18-tooth pinion.

Formula reference

GR       = Spur / Pinion
RPM_mot  = Kv × V_bat              (no-load approximation)
RPM_drv  = RPM_mot / GR
C_wheel  = π × D_mm / 1000         (metres)
v (m/s)  = RPM_drv × C_wheel / 60
v (km/h) = v (m/s) × 3.6

Inverse (find pinion for speed):
Pinion = (v_target_kmh / 3.6 × 60 × Spur) / (Kv × V_bat × π × D_m)

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