Speed Calculator Gear Ratio Tire Size

This calculator works out a vehicle’s road speed from its engine RPM, gear ratio, final drive ratio, and tire size. You enter the RPM, the gear ratio, the final drive (axle) ratio, and the tire diameter (or choose a common size), and you get the theoretical speed in km/h and mph. The result is the speed at that engine speed in that gear, assuming no clutch slip and no tire deformation.

Formula

The formula for road speed from gear ratio and tire size is:

Speed (km/h) = (RPM × Tire Diameter (m) × π × 60) / (Gear Ratio × Final Drive × 1000)

• RPM – engine revolutions per minute (no unit).
• Tire Diameter (m) – the overall diameter of the tire, in meters. For inches, multiply by 0.0254.
• Ï€ – approximately 3.14159.
• 60 – converts minutes to hours.
• Gear Ratio – the ratio of the selected gear (e.g., 4th gear = 1.00).
• Final Drive – the axle or differential ratio (e.g., 3.73:1).
• 1000 – converts meters to kilometers.

In imperial units the same formula becomes:

Speed (mph) = (RPM × Tire Diameter (in) × π × 60) / (Gear Ratio × Final Drive × 63360)

where 63360 is the number of inches in a mile.

Worked Example

Using the default values from the calculator: RPM = 6000, gear ratio = 1.00, final drive = 3.73, tire diameter = 26.5 inches.

1. Convert tire diameter to meters: 26.5 in × 0.0254 = 0.6731 m.
2. Tire circumference = 0.6731 × π = 2.114 m.
3. Wheel speed = RPM / (gear × final) = 6000 / (1.00 × 3.73) = 1608.6 wheel rpm.
4. Distance per minute = 2.114 × 1608.6 = 3400.6 m/min.
5. Speed = 3400.6 × 60 / 1000 = 204.0 km/h.

The calculator shows 204.0 km/h (126.8 mph). That matches.

How to Use the Calculator

Enter the engine speed (RPM) you want to calculate at, the gear ratio for that gear, the final drive ratio, and the tire diameter. The gear ratio is the ratio of the selected gear (e.g., 1st gear might be 3.5:1, 4th gear 1.0:1, 5th gear 0.85:1). The final drive is the axle ratio (e.g., 3.73:1). Tire diameter is the overall height of the tire, not the rim size. You can find it by measuring the tire or using the sidewall markings (see below).

The result is the theoretical speed at that RPM in that gear. It is the speed you would see on a flat road with no wind, no slip, and no tire compression. Real-world speed will be slightly lower due to tire squat and drivetrain losses, but the formula is very close for most vehicles.

How to Calculate Tire Diameter from Sidewall Markings

If you know the tire size, e.g., 225/45R17, you can compute the diameter:

• Section width = 225 mm.
• Aspect ratio = 45% of width = 0.45 × 225 = 101.25 mm sidewall height.
• Total tire height = 2 × sidewall + rim diameter = 2 × 101.25 + 17 × 25.4 = 202.5 + 431.8 = 634.3 mm.
• Diameter in inches = 634.3 / 25.4 = 24.97 in.

You can also measure directly with a tape measure around the center of the tire, then divide by Ï€ to get diameter. That method includes the slight deformation from the vehicle’s weight, which is actually more accurate for speed calculations.

Common Mistakes and Misconceptions

• Using rim size instead of tire diameter – a 17-inch rim is not the tire diameter; the tire adds sidewall height on both sides.
• Mixing units – if you use inches for diameter and km/h for speed, you must convert. The calculator does it for you.
• Forgetting the final drive – the gear ratio alone is not enough; the axle ratio multiplies the total reduction.
• Ignoring tire growth at speed – at high speed, tires can grow slightly (centrifugal force), increasing speed by 1-2%. This is usually negligible.
• Assuming the speedometer is accurate – speedometers are often calibrated to read slightly high (by law in many places), so the calculated speed may differ from the speedometer reading.

Why Use This Calculator?

This calculator is useful for:

• Choosing gear ratios – to see what RPM you’ll be at a given highway speed.
• Comparing tire sizes – larger tires increase speed per RPM but reduce acceleration and effective gearing.
• Planning engine mods – to estimate top speed or RPM at a target speed.
• Checking speedometer error – after changing tire size, the speedometer may be off; this calculator shows the true speed.
• Drag racing or track use – to find the optimal shift points or RPM at the end of a straight.

Remember, the calculator gives the theoretical speed. Aerodynamic drag, rolling resistance, and drivetrain losses mean the actual top speed may be lower, especially for high-speed runs.

Limitations and Accuracy

The formula assumes a rigid tire and no slip. In reality, tires deform at the contact patch, reducing the effective rolling radius by a few percent. Also, at very high speed, tires can grow due to centrifugal force, increasing the radius. The result is usually within 2-3% of actual speed, which is fine for most planning. For exact speedometer calibration, you should measure the tire’s rolling circumference under load (e.g., mark the tire and ground, roll one full turn, measure distance).

Frequently Asked Questions

How do I find my gear ratios?

Gear ratios are usually listed in the owner’s manual, service manual, or online for your specific make and model. You can also calculate them by counting teeth on the gears, but that requires disassembly. For a quick estimate, many online databases list gear ratios for common cars.

What if I change tire size? How much does it affect speed?

Speed is directly proportional to tire diameter. If you increase tire diameter by 5%, speed increases by 5% at the same RPM. For example, going from 26.5 in to 27.8 in (a 5% increase) will raise speed from 204 km/h to 214 km/h at 6000 RPM. This also means your speedometer will read lower than your actual speed.

Why is my calculated speed different from my speedometer?

Speedometers are often calibrated to read slightly higher than actual speed (to comply with regulations in many countries). Additionally, tire wear reduces diameter, and tire pressure affects rolling radius. The calculated speed is the true theoretical speed; the speedometer may have an intentional offset.

Can I use this for bicycles or motorcycles?

Yes, the same formula applies to any wheeled vehicle with a gear train. For bicycles, the final drive is the rear sprocket ratio, and the gear ratio is the chainring-to-sprocket ratio. For motorcycles, the final drive is the chain or shaft ratio. Just enter the appropriate values.

What is the difference between gear ratio and final drive?

The gear ratio is the ratio of the transmission gear you are in (e.g., 3rd gear). The final drive is the ratio of the differential (or chain/belt) that further reduces speed before the wheels. The total reduction is the product of both: total = gear × final.

How do I convert km/h to mph?

Multiply km/h by 0.621371 to get mph. The calculator shows both for convenience.

Related

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in rpm | Engine speed | rpm | 6000 | 0
in gr | Gear ratio | :1 | 1.00 | 0
in fd | Final drive ratio | :1 | 3.73 | 0
in td | Tire diameter | in | 26.5 | 0
out spd | Speed | km/h | (rpm * td * 0.0254 * pi * 60) / (gr * fd * 1000) | 1
out spm | Speed | mph | (rpm * td * pi * 60) / (gr * fd * 63360) | 1