What is the gear ratio speed formula?

Speed (mph) = (Engine RPM × π × tyre diameter in inches) / (transmission gear ratio × final drive ratio × 1056). The constant 1056 comes from unit conversion: 12 inches per foot × 5280 feet per mile / 60 minutes = 1056. Divide by 1.60934 to convert to km/h.

What is the difference between gear ratio and final drive ratio?

The transmission gear ratio is the ratio inside the gearbox for a particular gear (e.g. 1st gear might be 3.46:1). The final drive ratio is the fixed ratio in the differential or axle (e.g. 3.73:1). The overall ratio that the engine "sees" is the product of both: 3.46 × 3.73 = 12.91:1 in this example.

How do I find my car's gear ratios?

Gear ratios are listed in the vehicle's workshop manual, on manufacturer specification sheets, or on enthusiast forums specific to your model. You can also measure them empirically by recording engine RPM and road speed in a known gear on a flat road, then back-calculating using this calculator's 'Solve for gear ratio' mode.

How does tyre size affect speed calculations?

A larger tyre diameter means more distance per wheel revolution, so at the same RPM the vehicle moves faster. A P-metric size like 225/45R18 decodes as: section width 225 mm, sidewall = 225 × 0.45 = 101.25 mm each side, overall diameter = 18 inches + (2 × 101.25 / 25.4) = 25.97 inches. Even a 10 mm change in overall diameter shifts top speed and cruise RPM by roughly 1–2%.

What is a numerically high vs low final drive ratio?

A numerically high final drive (e.g. 4.30:1) multiplies torque more at the wheels, giving stronger acceleration but higher engine RPM at any given road speed. A numerically low ratio (e.g. 2.73:1) reduces torque multiplication but lowers cruise RPM, improving fuel economy. Long-ratio gearing suits motorway cruisers; short-ratio suits towing or track use.

Can I use this for motorcycles or trucks?

Yes. The underlying formula (speed = RPM × π × tyre diameter / (gear ratio × final drive × 1056)) is universal for any wheeled vehicle. For a motorcycle with a chain-drive primary and secondary reduction, multiply all ratios together to get the overall ratio, then enter that as a single-gear transmission with a final drive of 1:1 — or split them across the two fields.

Gear Ratio & Speed Calculator

A gear ratio and speed calculator that instantly tells you how fast your car, truck or motorcycle will travel at any engine RPM in any gear — or works backwards to find the RPM, gear ratio, final drive ratio or tyre diameter that achieves a target speed. Every calculation is shown step-by-step with units, and a full gear-by-gear speed table lets you compare your entire transmission at a glance.

How it works

Vehicle speed depends on four things: how fast the engine spins (RPM), how much the transmission reduces that spin (gear ratio), how much the axle reduces it further (final drive ratio) and how big the wheel and tyre are (tyre diameter). The core formula is:

speed (mph) = (RPM × π × D) / (GR × FDR × 1056)

where D is the outer tyre diameter in inches, GR is the transmission gear ratio for the selected gear, and FDR is the final drive (axle/differential) ratio. The constant 1056 combines the unit conversions: 12 in/ft × 5280 ft/mile ÷ 60 min = 1056.

Tyre diameter from P-metric size: A size like 195/65R15 means a section width of 195 mm and a sidewall height of 195 × 65 % = 126.75 mm. Each sidewall adds to the 15-inch rim, giving an overall diameter of 15 + (2 × 126.75 / 25.4) = 24.98 inches. The calculator does this conversion for you automatically.

Overall ratio: The transmission gear ratio and the final drive ratio multiply together. A 3rd-gear ratio of 1.29:1 combined with a 3.94:1 final drive gives an overall ratio of 1.29 × 3.94 = 5.08:1 — meaning the engine turns 5.08 times for every single wheel revolution in that gear.

Solving backwards: Rearrange the formula for any unknown. For RPM: RPM = speed × GR × FDR × 1056 / (π × D). For gear ratio: GR = RPM × π × D / (speed × FDR × 1056). For final drive: FDR = RPM × π × D / (speed × GR × 1056). For tyre diameter: D = speed × GR × FDR × 1056 / (RPM × π).

Worked example

A 2.0-litre hot hatch with a 5-speed gearbox, a 3.94:1 final drive and 195/65R15 tyres — what is its top speed in 5th gear at 6,500 RPM?

Tyre diameter: 15 + (2 × 195 × 0.65 / 25.4) = 24.98 inches
5th gear ratio: 0.76:1; final drive: 3.94:1; overall ratio = 0.76 × 3.94 = 2.99:1
Speed = (6500 × π × 24.98) / (0.76 × 3.94 × 1056) = 161.3 mph (259.6 km/h)

In 3rd gear (1.29:1) at 3,000 RPM the same car does:

Overall ratio = 1.29 × 3.94 = 5.08:1
Speed = (3000 × π × 24.98) / (5.08 × 1056) = 43.9 mph (70.7 km/h)

Gear	Ratio	Speed at 3,000 RPM	Speed at 6,500 RPM
1st	3.46	16.4 mph	35.5 mph
2nd	1.94	29.2 mph	63.2 mph
3rd	1.29	43.9 mph	95.1 mph
4th	0.97	58.4 mph	126.5 mph
5th	0.76	74.5 mph	161.4 mph

Formula note

The 1056 constant is exact: 5280 ft/mile × 12 in/ft / 60 min/hr = 1056. Some sources use 336 for km/h directly; this calculator converts via mph × 1.60934 to maintain precision. Wheel RPM = engine RPM / (GR × FDR). Tyre circumference (in) = π × D. Speed (in/min) = wheel RPM × circumference. Speed (mph) = speed (in/min) × 60 / 63,360.