Gear Ratio Speed Calculator MPH: How It Works

This calculator works out the theoretical top speed in miles per hour (MPH) from your gear ratio, tire diameter, and engine RPM. You enter the number of teeth on the engine-side (drive) gear and the wheel-side (driven) gear, the tire diameter in inches, and the engine speed in RPM; it returns the vehicle speed in MPH. The result is a pure drivetrain calculation that ignores wind resistance, drivetrain losses, and tire slip, so real-world speed will be slightly lower.

Formula

The formula used by the calculator is:

MPH = (RPM × Tire Diameter (in) × π) ÷ (Gear Ratio × 1056)

Where:

• RPM = engine revolutions per minute (unit: rev/min)
• Tire Diameter = overall diameter of the tire in inches (unit: in)
• Ï€ (pi) ≈ 3.14159
• Gear Ratio = number of driven gear teeth ÷ number of drive gear teeth (unitless)
• 1056 = conversion factor that combines inches-to-miles (1 mile = 63,360 in) and minutes-to-hours (60 min/hr). The math: 63,360 ÷ (Ï€ × 60) ≈ 336.13, but because we multiply by Ï€ in the numerator, the constant becomes 63,360 ÷ 60 = 1056. This is why the formula works cleanly.

If you prefer metric (km/h), replace 1056 with 336.13 and use tire diameter in inches (or convert to cm and use a different constant). The calculator above outputs MPH directly.

Worked Example

Using the default values from the calculator:

• Drive gear teeth = 20
• Driven gear teeth = 40
• Tire diameter = 26 inches
• Engine RPM = 6000

First, compute the gear ratio:

Gear Ratio = 40 ÷ 20 = 2.0

Now apply the formula:

MPH = (6000 × 26 × 3.14159) ÷ (2.0 × 1056)

Step by step:

1. 6000 × 26 = 156,000
2. 156,000 × 3.14159 = 490,088.4
3. 2.0 × 1056 = 2112
4. 490,088.4 ÷ 2112 = 232.05

The calculator rounds to 1 decimal place, so the result is 232.0 MPH. That is the theoretical top speed at 6000 RPM with a 2:1 gear ratio and 26-inch tires. In reality, aerodynamic drag and rolling resistance would limit a typical vehicle to far less than this.

What This Calculator Is For

This calculator is most useful for hobbyists and engineers who are designing or modifying vehicles—go-karts, mini bikes, bicycles with engine kits, RC cars, or even full-size cars when planning gear changes. It answers questions like “If I install a smaller rear sprocket, how much faster will I go at the same RPM?� or “What RPM do I need to reach 60 MPH with my current tires and gearing?�

It is also useful for comparing final drive ratios. The final drive ratio is the gear ratio multiplied by any transmission gear ratio. For example, if your car’s 4th gear has a ratio of 1.0 and your differential has a ratio of 3.5, the total ratio is 3.5. You would enter 1 as the drive gear and 3.5 as the driven gear (or vice versa, depending on which side you call drive). The calculator treats the ratio as driven ÷ drive, so a larger number means more reduction (slower speed, more torque).

How to Choose the Right Gear Ratio

Choosing a gear ratio depends on your goal—top speed, acceleration, or towing. Here are the practical steps:

• Decide your target top speed and the RPM at which you want to reach it (often near peak power).
• Measure the tire diameter (in inches) by measuring from the ground to the center of the wheel and doubling it, or by checking the tire sidewall (e.g., a 26-inch wheel).
• Use the calculator to try different sprocket tooth counts. For example, if you want 70 MPH at 6000 RPM with 26-inch tires, you need a gear ratio of about 1.47 (i.e., driven ÷ drive = 1.47). That means if your drive sprocket has 20 teeth, the driven sprocket should have about 29 teeth.
• Remember that a numerically lower gear ratio (e.g., 2.0 vs. 3.0) gives higher top speed but less acceleration. A higher ratio (e.g., 4.0) gives quicker acceleration but a lower top speed.

Always check that the engine can actually reach the RPM you used. If the engine’s redline is 6000 RPM, using 8000 RPM in the calculator will give a speed the engine cannot achieve.

Common Mistakes and Misconceptions

The most common mistake is mixing up the drive and driven gears. The drive gear is the one connected to the engine or motor; the driven gear is the one on the wheel or axle. Swapping them inverts the ratio and gives a wildly wrong speed.

Another frequent error is using wheel diameter instead of tire diameter. The tire diameter includes the rubber, not just the rim. For example, a 17-inch rim with a 2-inch sidewall tire has a total diameter of 21 inches, not 17. Using the rim size alone will underestimate speed by about 20%.

A third misconception is that the result is the actual top speed. The formula assumes no power limitations, no drag, no rolling resistance, and no clutch slip. In reality, a vehicle will only reach this speed if the engine has enough power to overcome all resistances. For a go-kart, the actual speed might be 80-90% of the theoretical value. For a bicycle with a small engine, it might be even less.

Also, the calculator only works for a single gear. If your vehicle has multiple gears, you must calculate each gear separately. The top speed in each gear is the MPH at the RPM you enter, but the engine may not be able to reach that RPM in a lower gear due to power limitations.

Limits of the Formula

This is a kinematic formula—it only relates rotational speed to linear speed. It does not account for:

• Engine torque and power curve – the engine may not produce enough power at the RPM you entered to overcome drag.
• Aerodynamic drag – increases with the square of speed, so at high speeds it dramatically reduces actual top speed.
• Rolling resistance and drivetrain friction – these always reduce efficiency.
• Tire growth – at high speed, tires can expand slightly, which increases the effective diameter and thus the actual speed.
• Transmission losses – typical losses are 10-15% for a chain drive, more for a gearbox.

For a rough real-world estimate, multiply the theoretical MPH by 0.85 to 0.95 for a well-maintained vehicle. For a more accurate figure, you would need to measure the actual speed with a GPS or a speedometer.

Frequently Asked Questions

What is the 1056 constant in the formula? It converts inches per minute to miles per hour. There are 63,360 inches in a mile and 60 minutes in an hour, so 63,360 ÷ 60 = 1056. The formula multiplies the circumference (π × diameter) by RPM to get inches per minute, then divides by 1056 to get MPH.

How do I calculate MPH with a gear ratio of 3.73 and 33-inch tires at 2500 RPM? Using the formula: MPH = (2500 × 33 × 3.14159) ÷ (3.73 × 1056) = 259,181 ÷ 3938.88 ≈ 65.8 MPH. That is the theoretical speed; actual speed will be lower.

Does this calculator work for bicycles? Yes, but you must account for the chainring and cassette. Enter the chainring teeth as the drive gear and the cassette teeth as the driven gear. Also, the tire diameter for a typical road bike is about 26-27 inches. The result is the speed at the given cadence (RPM of the pedals).

Why does my car’s actual top speed differ from the calculated value? Because the calculation ignores aerodynamic drag, which grows with the square of speed. At 100 MPH, drag is four times greater than at 50 MPH. The engine simply may not have enough power to overcome that drag, so the car tops out at a lower speed than the gear ratio suggests.

Can I use this for an RC car? Yes, but RC cars often have very small tires (measured in millimeters) and high RPM brushless motors. Convert tire diameter to inches (1 inch = 25.4 mm) and use the motor’s no-load RPM for a theoretical maximum. Actual speed will be lower due to friction and battery voltage drop.

What if I have a two-stage reduction (e.g., engine to jackshaft, then jackshaft to wheel)? Multiply the two ratios together to get the total gear ratio. For example, if the first stage is 2:1 and the second is 3:1, the total ratio is 6:1. Enter that as the driven/drive ratio (e.g., drive = 1, driven = 6).

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in drive | Drive gear teeth | teeth | 20 | 1
in driven | Driven gear teeth | teeth | 40 | 1
in tire | Tire diameter | in | 26 | 1
in rpm | Engine speed | RPM | 6000 | 0
out mph | Vehicle speed | MPH | (rpm * tire * pi) / (driven / drive * 1056) | 1
out gear_ratio | Gear ratio | :1 | driven / drive | 2