{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"gear-ratio-speed"},"result":{"entity_type":"calculator","id":"gear-ratio-speed","calculator_id":"gear-ratio-speed","canonical_url":"https://tttkmbb.com/engineering/gear-ratio-speed","name":"Gear Ratio and Speed Calculator","title":"Gear Ratio Speed Calculator – Road Speed from Engine RPM, Gear Ratio, Final Drive and Tire Size, or RPM at a Speed","category":"engineering","category_name":"Engineering & Automotive","tool_name":"calculate_gear_ratio_speed","featured_mcp_tool":false,"description":"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.","use_when":"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).","inputs":[{"name":"engine_rpm","label":"Engine speed","type":"number","unit":"rpm","required":true,"max":30000,"exclusive_min":0,"description":"Engine (or motor) crankshaft speed in revolutions per minute.","example":3000},{"name":"gear_ratio","label":"Transmission gear ratio","type":"number","required":true,"max":50,"exclusive_min":0,"description":"Ratio 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.","example":1},{"name":"final_drive_ratio","label":"Final drive ratio","type":"number","required":true,"max":50,"exclusive_min":0,"description":"Differential / axle ratio (e.g. 3.73).","example":3.73},{"name":"tire_diameter_mm","label":"Tire diameter","type":"number","unit":"mm","required":false,"max":3000,"exclusive_min":0,"description":"Overall tire diameter in millimetres (inches × 25.4). Give either tire_diameter_mm or tire_code.","example":634.3},{"name":"tire_code","label":"Tire code","type":"string","required":false,"description":"Metric tire size such as 225/45R17, used to compute the diameter instead of tire_diameter_mm."},{"name":"speed_km_h","label":"Road speed for rpm lookup","type":"number","unit":"km/h","required":false,"max":600,"exclusive_min":0,"description":"Optional road speed; enables the engine rpm at that speed in the same gear.","example":100}],"outputs":[{"name":"overall_ratio","label":"Overall ratio","type":"number","decimals":4,"description":"gear_ratio × final_drive_ratio (engine revolutions per wheel revolution)."},{"name":"wheel_rpm","label":"Wheel speed","type":"number","unit":"rpm","decimals":2,"description":"engine_rpm / overall_ratio."},{"name":"tire_diameter_mm","label":"Tire diameter used","type":"number","unit":"mm","decimals":1,"description":"Overall tire diameter."},{"name":"tire_circumference_m","label":"Tire circumference","type":"number","unit":"m","decimals":4,"description":"π × diameter."},{"name":"speed_km_h","label":"Road speed","type":"number","unit":"km/h","decimals":2,"description":"wheel_rpm × circumference × 60 / 1000."},{"name":"speed_mph","label":"Road speed","type":"number","unit":"mph","decimals":2,"description":"speed_km_h / 1.609344."},{"name":"km_h_per_1000_rpm","label":"Speed per 1000 rpm","type":"number","unit":"km/h","decimals":2,"description":"Road speed gained per 1000 engine rpm in this gear."},{"name":"mph_per_1000_rpm","label":"Speed per 1000 rpm","type":"number","unit":"mph","decimals":2,"description":"Same in mph."},{"name":"rpm_at_speed","label":"Engine rpm at speed_km_h","type":"number","unit":"rpm","decimals":0,"description":"Engine speed needed for the given road speed in this gear (only when speed_km_h is given)."}],"input_schema":{"type":"object","properties":{"engine_rpm":{"description":"Engine (or motor) crankshaft speed in revolutions per minute. Unit: rpm.","type":"number","maximum":30000,"exclusiveMinimum":0,"examples":[3000],"x-unit":"rpm"},"gear_ratio":{"description":"Ratio 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.","type":"number","maximum":50,"exclusiveMinimum":0,"examples":[1]},"final_drive_ratio":{"description":"Differential / axle ratio (e.g. 3.73).","type":"number","maximum":50,"exclusiveMinimum":0,"examples":[3.73]},"tire_diameter_mm":{"description":"Overall tire diameter in millimetres (inches × 25.4). Give either tire_diameter_mm or tire_code. Unit: mm.","type":"number","maximum":3000,"exclusiveMinimum":0,"examples":[634.3],"x-unit":"mm"},"tire_code":{"description":"Metric tire size such as 225/45R17, used to compute the diameter instead of tire_diameter_mm.","type":"string"},"speed_km_h":{"description":"Optional road speed; enables the engine rpm at that speed in the same gear. Unit: km/h.","type":"number","maximum":600,"exclusiveMinimum":0,"examples":[100],"x-unit":"km/h"}},"additionalProperties":false,"required":["engine_rpm","gear_ratio","final_drive_ratio"]},"output_schema":{"type":"object","properties":{"overall_ratio":{"description":"gear_ratio × final_drive_ratio (engine revolutions per wheel revolution).","type":"number"},"wheel_rpm":{"description":"engine_rpm / overall_ratio. Unit: rpm.","type":"number","x-unit":"rpm"},"tire_diameter_mm":{"description":"Overall tire diameter. Unit: mm.","type":"number","x-unit":"mm"},"tire_circumference_m":{"description":"π × diameter. Unit: m.","type":"number","x-unit":"m"},"speed_km_h":{"description":"wheel_rpm × circumference × 60 / 1000. Unit: km/h.","type":"number","x-unit":"km/h"},"speed_mph":{"description":"speed_km_h / 1.609344. Unit: mph.","type":"number","x-unit":"mph"},"km_h_per_1000_rpm":{"description":"Road speed gained per 1000 engine rpm in this gear. Unit: km/h.","type":"number","x-unit":"km/h"},"mph_per_1000_rpm":{"description":"Same in mph. Unit: mph.","type":"number","x-unit":"mph"},"rpm_at_speed":{"description":"Engine speed needed for the given road speed in this gear (only when speed_km_h is given). Unit: rpm.","type":"number","x-unit":"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","method":"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.","sources":[{"name":"Wikipedia – Gear train (gear ratio)","url":"https://en.wikipedia.org/wiki/Gear_train","type":"reference","retrieved_at":"2026-09-24"},{"name":"Wikipedia – Tire code","url":"https://en.wikipedia.org/wiki/Tire_code","type":"reference","retrieved_at":"2026-09-24"}],"freshness":{"type":"static","max_age_seconds":null,"note":"Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output."},"examples":[{"name":"3000 rpm, 1:1 gear, 3.73 final drive, 634.3 mm tire, rpm at 100 km/h","inputs":{"engine_rpm":3000,"gear_ratio":1,"final_drive_ratio":3.73,"tire_diameter_mm":634.3,"speed_km_h":100},"expected":{"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},"url":"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"},{"name":"2500 rpm in 0.8 overdrive, 4.10 final drive, 205/55R16","inputs":{"engine_rpm":2500,"gear_ratio":0.8,"final_drive_ratio":4.1,"tire_code":"205/55R16"},"expected":{"overall_ratio":3.28,"tire_diameter_mm":631.9,"speed_km_h":90.79,"speed_mph":56.41},"url":"https://tttkmbb.com/api/v1/calculate/gear-ratio-speed?engine_rpm=2500&gear_ratio=0.8&final_drive_ratio=4.1&tire_code=205%2F55R16"}],"faq":[{"q":"Where do I find the gear and final drive ratios?","a":"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."},{"q":"How do bigger tires change the result?","a":"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."}],"tags":["gear ratio calculator","rpm to speed","speed at rpm","final drive ratio","tire size gearing"],"related":[{"calculator_id":"tire-size","reason":"Tire diameter from the sidewall code."},{"calculator_id":"horsepower-torque-rpm","reason":"Engine power and torque at the rpm."},{"calculator_id":"speed","reason":"Convert road speed between units."}],"links":{"html":"https://tttkmbb.com/engineering/gear-ratio-speed","markdown":"https://tttkmbb.com/engineering/gear-ratio-speed.md","json":"https://tttkmbb.com/engineering/gear-ratio-speed.json","api":"https://tttkmbb.com/api/v1/calculate/gear-ratio-speed","schema":"https://tttkmbb.com/api/v1/calculators/gear-ratio-speed","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T01:47:12Z","next_actions":[{"tool":"run_calculator","calculator_id":"gear-ratio-speed","reason":"Run Gear Ratio and Speed Calculator with the inputs above."}],"links":{"markdown":"https://tttkmbb.com/engineering/gear-ratio-speed.md"}}