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RC Gear Ratio Calculator

Compute FDR, Wheel RPM and Top Speed

Quickly determine final drive ratio (FDR), wheel RPM and speed for both brushless electric and nitro setups. Tune gearing for optimal performance and temperature control.

"Gearing is strategy. The right FDR makes the power usable and the RC car consistent."
- Championship RC Racer

RC Gear Ratio Calculator Tool

Enter your parameters to calculate FDR, wheel RPM and speed (MPH/Km/h).

Which inputs are required?

  • FDR only: Pinion, Spur, Internal Gear Ratio
  • Wheel RPM: FDR + Motor RPM (Electric: KV × Voltage; Nitro: Engine RPM)
  • Speed: Wheel RPM + Tire Diameter (inches)

Need help finding values? See how to get these parameters

RPM/V
V
inches

Results:

0
FDR: -, Wheel RPM: -

How to Calculate RC Gear Ratio and Speed

RC Brushless Electric Gear Ratio Calculation Examples:

KV Voltage Pinion Spur Internal Tire (in) Speed (MPH/Km/h)
2200 11.1 (3S) 18 54 2.6 2.6 43.3 / 69.7
3500 7.4 (2S) 20 80 2.6 2.2 34.7 / 55.8
1900 14.8 (4S) 16 50 2.85 3.0 48.9 / 78.7
2650 11.1 (3S) 22 72 2.85 2.4 45.5 / 73.2
3000 7.4 (2S) 19 76 2.5 2.6 39.9 / 64.2

RC Brushless Gear Ratio Speed Chart Table


RC Brushless Gear Ratio Speed Calculation Formulas:

Motor RPM = KV × Voltage

FDR = (Spur ÷ Pinion) × Internal Ratio

Wheel RPM = Motor RPM ÷ FDR

Speed (MPH) = Wheel RPM × Tire Diameter (in) × 3.14159 × 60 × 1.578 ÷ 100000


Example 1: 2200KV Motor with 3S Battery

  • Motor KV: 2200
  • Battery Voltage: 11.1V (3S)
  • Pinion Gear Teeth: 18T
  • Spur Gear Teeth: 54T
  • Internal Gear Ratio: 2.6
  • Tire Diameter: 2.6 inches

Step-by-step calculation:

  1. Motor RPM = 2200 × 11.1 = 24,420 RPM
  2. FDR = (54 ÷ 18) × 2.6 = 7.8
  3. Wheel RPM = 24,420 ÷ 7.8 = 3,131 RPM
  4. Speed (MPH) = 3,131 × 2.6 × 3.14159 × 60 × 1.578 ÷ 100000 = 43.3 MPH
  5. Speed (Km/h) = 43.3 × 1.609344 = 69.7 Km/h

Result: FDR = 7.8, Wheel RPM = 3,131, Speed = 43.3 MPH / 69.7 Km/h

Example 2: 3500KV Motor with 2S Battery

  • Motor KV: 3500
  • Battery Voltage: 7.4V (2S)
  • Pinion Gear Teeth: 20T
  • Spur Gear Teeth: 80T
  • Internal Gear Ratio: 2.6
  • Tire Diameter: 2.2 inches

Step-by-step calculation:

  1. Motor RPM = 3500 × 7.4 = 25,900 RPM
  2. FDR = (80 ÷ 20) × 2.6 = 10.4
  3. Wheel RPM = 25,900 ÷ 10.4 = 2,490 RPM
  4. Speed (MPH) = 2,490 × 2.2 × 3.14159 × 60 × 1.578 ÷ 100000 = 34.7 MPH
  5. Speed (Km/h) = 34.7 × 1.609344 = 55.8 Km/h

Result: FDR = 10.4, Wheel RPM = 2,490, Speed = 34.7 MPH / 55.8 Km/h


RC Nitro/Gas Gear Ratio Calculation Examples:

Engine RPM Pinion Spur Internal Tire (in) Speed (MPH/Km/h)
30,000 18 54 2.6 2.6 29.7 / 47.8
32,000 20 80 2.6 2.2 20.1 / 32.4
28,000 16 50 2.85 3.0 28.0 / 45.1
35,000 22 72 2.85 2.4 26.8 / 43.1
38,000 19 76 2.5 2.8 31.7 / 51.0

RC Nitro/Gas Gear Ratio Speed Chart Table


RC Nitro/Gas Gear Ratio Speed Calculation Formulas:

Motor RPM = Engine RPM

FDR = (Spur ÷ Pinion) × Internal Ratio

Wheel RPM = Motor RPM ÷ FDR

Speed (MPH) = Wheel RPM × Tire Diameter (in) × 3.14159 × 60 × 1.578 ÷ 100000


Example 1: .21 Engine with 18T/54T Gears

  • Engine RPM: 30,000
  • Pinion Gear Teeth: 18T
  • Spur Gear Teeth: 54T
  • Internal Gear Ratio: 2.6
  • Tire Diameter: 2.6 inches

Step-by-step calculation:

  1. Motor RPM = Engine RPM = 30,000
  2. FDR = (54 ÷ 18) × 2.6 = 7.8
  3. Wheel RPM = 30,000 ÷ 7.8 = 3,846 RPM
  4. Speed (MPH) = 3,846 × 2.6 × 3.14159 × 60 × 1.578 ÷ 100000 = 29.7 MPH
  5. Speed (Km/h) = 29.7 × 1.609344 = 47.8 Km/h

Result: FDR = 7.8, Wheel RPM = 3,846, Speed = 29.7 MPH / 47.8 Km/h

Example 2: .28 Engine with 20T/80T Gears

  • Engine RPM: 32,000
  • Pinion Gear Teeth: 20T
  • Spur Gear Teeth: 80T
  • Internal Gear Ratio: 2.6
  • Tire Diameter: 2.2 inches

Step-by-step calculation:

  1. Motor RPM = Engine RPM = 32,000
  2. FDR = (80 ÷ 20) × 2.6 = 10.4
  3. Wheel RPM = 32,000 ÷ 10.4 = 3,077 RPM
  4. Speed (MPH) = 3,077 × 2.2 × 3.14159 × 60 × 1.578 ÷ 100000 = 20.1 MPH
  5. Speed (Km/h) = 20.1 × 1.609344 = 32.4 Km/h

Result: FDR = 10.4, Wheel RPM = 3,077, Speed = 20.1 MPH / 32.4 Km/h


How to Use This Calculator:

  1. Enter Pinion, Spur and Internal Gear Ratio to get FDR.
  2. For Electric, input KV and Voltage to compute Motor RPM; for Nitro, enter Engine RPM.
  3. Provide Tire Diameter (inches) to calculate final speed.
  4. Click Calculate and review FDR, Wheel RPM and Speed outputs.

Pro Tips from Top Racers:

  • Shorter gearing (higher FDR) improves torque and temps; taller gearing increases top speed but risks heat.
  • Adjust 1 tooth at a time on pinion and recheck temps after a full run.
  • Measure real tire diameter at ride height; worn or belted tires change effective speed.
  • Target motor temps 65–75°C (electric) and consistent tune for nitro before testing.
  • Use GPS logs to validate results and fine-tune internal ratio assumptions.

How to find these parameters? (Electric)

  • Motor KV: Printed on motor or listed on the product page/spec sheet.
  • Battery Voltage: Nominal LiPo value (2S 7.4V, 3S 11.1V, 4S 14.8V).
  • Pinion/Spur Teeth: Count gear teeth or read the printed numbers.
  • Internal Gear Ratio: Found in the chassis manual/manufacturer specs.
  • Tire Diameter (inches): Measure with calipers across the tire; convert to inches.

How to find these parameters? (Nitro/Gas)

  • Engine RPM: From engine specs or measured with a tachometer after warm-up.
  • Pinion/Spur Teeth: Count tooth numbers directly.
  • Internal Gear Ratio: See vehicle manual or brand’s website.
  • Tire Diameter (inches): Measure the actual diameter at ride height.