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Gear Ratio and Speed Calculator

Computes vehicle road speed in km/h and mph from engine rpm, transmission gear ratio, final-drive ratio and tire diameter (or tire code), plus the overall ratio, wheel rpm, speed per 1000 rpm and, optionally, the engine rpm at a given road speed.

When to use

You want to know how fast a vehicle travels at a given rpm in a given gear, what rpm it turns at highway speed, or how a tire or final-drive change shifts the gearing.

Do not use when: You need bicycle gearing (chainring/cog teeth and cadence), or torque and power at the engine (use horsepower-torque-rpm).

Formula

overall_ratio = gear_ratio × final_drive_ratio; wheel_rpm = engine_rpm / overall_ratio; speed_km_h = wheel_rpm × π × tire_diameter_mm / 1000 × 60 / 1000; rpm_at_speed = engine_rpm × speed_km_h / speed_km_h_computed

Assumes no tire slip and the unloaded nominal diameter; the loaded rolling circumference is typically 2–3 % smaller, so real speeds are slightly lower. Transfer-case low range multiplies the overall ratio further.

Inputs

ParameterTypeUnitRequiredDescription
engine_rpmnumberrpmyesEngine (or motor) crankshaft speed in revolutions per minute. Range: > 0, ≤ 30000
gear_rationumberyesRatio of the selected gear (e.g. 3.5 first gear, 1.0 direct, 0.75 overdrive); use 1 for a direct-drive EV reduction already included in final_drive_ratio. Range: > 0, ≤ 50
final_drive_rationumberyesDifferential / axle ratio (e.g. 3.73). Range: > 0, ≤ 50
tire_diameter_mmnumbermmnoOverall tire diameter in millimetres (inches × 25.4). Give either tire_diameter_mm or tire_code. Range: > 0, ≤ 3000
tire_codestringnoMetric tire size such as 225/45R17, used to compute the diameter instead of tire_diameter_mm.
speed_km_hnumberkm/hnoOptional road speed; enables the engine rpm at that speed in the same gear. Range: > 0, ≤ 600

Outputs

OutputTypeUnitDescription
overall_rationumbergear_ratio × final_drive_ratio (engine revolutions per wheel revolution).
wheel_rpmnumberrpmengine_rpm / overall_ratio.
tire_diameter_mmnumbermmOverall tire diameter.
tire_circumference_mnumbermπ × diameter.
speed_km_hnumberkm/hwheel_rpm × circumference × 60 / 1000.
speed_mphnumbermphspeed_km_h / 1.609344.
km_h_per_1000_rpmnumberkm/hRoad speed gained per 1000 engine rpm in this gear.
mph_per_1000_rpmnumbermphSame in mph.
rpm_at_speednumberrpmEngine speed needed for the given road speed in this gear (only when speed_km_h is given).

Example

3000 rpm, 1:1 gear, 3.73 final drive, 634.3 mm tire, rpm at 100 km/h: {"engine_rpm":3000,"gear_ratio":1,"final_drive_ratio":3.73,"tire_diameter_mm":634.3,"speed_km_h":100} → {"overall_ratio":3.73,"wheel_rpm":804.29,"tire_circumference_m":1.9927,"speed_km_h":96.16,"speed_mph":59.75,"km_h_per_1000_rpm":32.05,"rpm_at_speed":3120}

2500 rpm in 0.8 overdrive, 4.10 final drive, 205/55R16: {"engine_rpm":2500,"gear_ratio":0.8,"final_drive_ratio":4.1,"tire_code":"205/55R16"} → {"overall_ratio":3.28,"tire_diameter_mm":631.9,"speed_km_h":90.79,"speed_mph":56.41}

GET https://tttkmbb.com/api/v1/calculate/gear-ratio-speed?engine_rpm=3000&gear_ratio=1&final_drive_ratio=3.73&tire_diameter_mm=634.3&speed_km_h=100

Machine access

Sources

FAQ

Where do I find the gear and final drive ratios?

In the vehicle's technical specifications or workshop manual; the axle ratio is often on a door or axle tag. Automatic transmissions with torque converters slip below lock-up, so real rpm is higher at low speed.

How do bigger tires change the result?

Speed scales with tire diameter: a 3 % larger tire gives 3 % more speed at the same rpm and 3 % lower rpm at the same speed, like a numerically lower final drive.

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