{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"tire-size"},"result":{"entity_type":"calculator","id":"tire-size","calculator_id":"tire-size","canonical_url":"https://tttkmbb.com/engineering/tire-size","name":"Tire Size Calculator","title":"Tire Size Calculator – Diameter, Sidewall, Circumference and Revolutions per km/mile from a Tire Code, with Size Comparison","category":"engineering","category_name":"Engineering & Automotive","tool_name":"calculate_tire_size","featured_mcp_tool":false,"description":"Parses a metric tire code such as 225/45R17 into section width, aspect ratio and rim size, computes sidewall height, overall diameter, circumference and revolutions per kilometre and mile, and optionally compares a second size (diameter difference and speedometer error).","use_when":"You want the dimensions of a tire from its sidewall code, or want to check whether an alternative size keeps the overall diameter (and speedometer) within a few percent.","do_not_use_when":"The tire uses a flotation size (31x10.50R15) or an old alphanumeric code, or you need road speed from rpm and gearing (use gear-ratio-speed).","inputs":[{"name":"tire_code","label":"Tire code","type":"string","required":true,"description":"Metric tire size as printed on the sidewall: width mm / aspect ratio % R rim inches, e.g. 225/45R17 (prefixes like P or LT and suffixes like 91W are ignored).","example":"225/45R17"},{"name":"compare_tire_code","label":"Compare with tire code","type":"string","required":false,"description":"Optional second (new) tire size to compare against tire_code (the original), e.g. 235/40R18."}],"outputs":[{"name":"width_mm","label":"Section width","type":"number","unit":"mm","decimals":0,"description":"Nominal section width."},{"name":"aspect_ratio_percent","label":"Aspect ratio","type":"number","unit":"%","decimals":0,"description":"Sidewall height as a percentage of the width."},{"name":"rim_diameter_in","label":"Rim diameter","type":"number","unit":"in","decimals":1,"description":"Wheel diameter in inches."},{"name":"sidewall_height_mm","label":"Sidewall height","type":"number","unit":"mm","decimals":2,"description":"width × aspect ratio / 100."},{"name":"overall_diameter_mm","label":"Overall diameter","type":"number","unit":"mm","decimals":1,"description":"2 × sidewall + rim × 25.4."},{"name":"overall_diameter_in","label":"Overall diameter","type":"number","unit":"in","decimals":2,"description":"Overall diameter in inches."},{"name":"circumference_mm","label":"Circumference","type":"number","unit":"mm","decimals":1,"description":"π × overall diameter."},{"name":"revolutions_per_km","label":"Revolutions per km","type":"number","unit":"rev/km","decimals":1,"description":"1,000,000 / circumference_mm."},{"name":"revolutions_per_mile","label":"Revolutions per mile","type":"number","unit":"rev/mile","decimals":1,"description":"1,609,344 / circumference_mm."},{"name":"compare_overall_diameter_mm","label":"Compared tire diameter","type":"number","unit":"mm","decimals":1,"description":"Overall diameter of compare_tire_code (only when given)."},{"name":"diameter_difference_mm","label":"Diameter difference","type":"number","unit":"mm","decimals":1,"description":"compare − original (only when compare_tire_code is given)."},{"name":"diameter_difference_percent","label":"Diameter difference","type":"number","unit":"%","decimals":2,"description":"(compare − original) / original × 100; keep within about ±3 %."},{"name":"actual_speed_at_indicated_100","label":"Actual speed when speedometer shows 100","type":"number","unit":"km/h or mph","decimals":2,"description":"With the compared tire fitted, true speed when the speedometer (calibrated for the original) reads 100 (only when compare_tire_code is given)."},{"name":"size_change_assessment","label":"Size change assessment","type":"string","decimals":4,"description":"Whether the diameter change stays within the common ±3 % guideline (only when compare_tire_code is given)."}],"input_schema":{"type":"object","properties":{"tire_code":{"description":"Metric tire size as printed on the sidewall: width mm / aspect ratio % R rim inches, e.g. 225/45R17 (prefixes like P or LT and suffixes like 91W are ignored).","type":"string","examples":["225/45R17"]},"compare_tire_code":{"description":"Optional second (new) tire size to compare against tire_code (the original), e.g. 235/40R18.","type":"string"}},"additionalProperties":false,"required":["tire_code"]},"output_schema":{"type":"object","properties":{"width_mm":{"description":"Nominal section width. Unit: mm.","type":"number","x-unit":"mm"},"aspect_ratio_percent":{"description":"Sidewall height as a percentage of the width. Unit: %.","type":"number","x-unit":"%"},"rim_diameter_in":{"description":"Wheel diameter in inches. Unit: in.","type":"number","x-unit":"in"},"sidewall_height_mm":{"description":"width × aspect ratio / 100. Unit: mm.","type":"number","x-unit":"mm"},"overall_diameter_mm":{"description":"2 × sidewall + rim × 25.4. Unit: mm.","type":"number","x-unit":"mm"},"overall_diameter_in":{"description":"Overall diameter in inches. Unit: in.","type":"number","x-unit":"in"},"circumference_mm":{"description":"π × overall diameter. Unit: mm.","type":"number","x-unit":"mm"},"revolutions_per_km":{"description":"1,000,000 / circumference_mm. Unit: rev/km.","type":"number","x-unit":"rev/km"},"revolutions_per_mile":{"description":"1,609,344 / circumference_mm. Unit: rev/mile.","type":"number","x-unit":"rev/mile"},"compare_overall_diameter_mm":{"description":"Overall diameter of compare_tire_code (only when given). Unit: mm.","type":"number","x-unit":"mm"},"diameter_difference_mm":{"description":"compare − original (only when compare_tire_code is given). Unit: mm.","type":"number","x-unit":"mm"},"diameter_difference_percent":{"description":"(compare − original) / original × 100; keep within about ±3 %. Unit: %.","type":"number","x-unit":"%"},"actual_speed_at_indicated_100":{"description":"With the compared tire fitted, true speed when the speedometer (calibrated for the original) reads 100 (only when compare_tire_code is given). Unit: km/h or mph.","type":"number","x-unit":"km/h or mph"},"size_change_assessment":{"description":"Whether the diameter change stays within the common ±3 % guideline (only when compare_tire_code is given).","type":"string"}}},"formula":"sidewall_height_mm = width_mm × aspect_ratio_percent / 100; overall_diameter_mm = 2 × sidewall_height_mm + rim_diameter_in × 25.4; circumference_mm = π × overall_diameter_mm; revolutions_per_km = 1e6 / circumference_mm; actual_speed_at_indicated_100 = 100 × compare_diameter / original_diameter","method":"Nominal dimensions from the code (ISO 4000 / ETRTO); actual tires vary by a few millimetres between brands and the loaded rolling circumference is 2–3 % smaller than the unloaded value.","sources":[{"name":"Wikipedia – Tire code","url":"https://en.wikipedia.org/wiki/Tire_code","type":"reference","retrieved_at":"2026-09-24"},{"name":"ETRTO – European Tyre and Rim Technical Organisation standards manual","url":"https://www.etrto.org/","type":"standard","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":"225/45R17","inputs":{"tire_code":"225/45R17"},"expected":{"width_mm":225,"aspect_ratio_percent":45,"rim_diameter_in":17,"sidewall_height_mm":101.25,"overall_diameter_mm":634.3,"overall_diameter_in":24.97,"circumference_mm":1992.7,"revolutions_per_km":501.8,"revolutions_per_mile":807.6},"url":"https://tttkmbb.com/api/v1/calculate/tire-size?tire_code=225%2F45R17"},{"name":"205/55R16 replaced by 225/45R17","inputs":{"tire_code":"205/55R16","compare_tire_code":"225/45R17"},"expected":{"overall_diameter_mm":631.9,"compare_overall_diameter_mm":634.3,"diameter_difference_mm":2.4,"diameter_difference_percent":0.38,"actual_speed_at_indicated_100":100.38,"size_change_assessment":"Within the ±3 % diameter guideline"},"url":"https://tttkmbb.com/api/v1/calculate/tire-size?tire_code=205%2F55R16&compare_tire_code=225%2F45R17"}],"faq":[{"q":"What do the numbers in 225/45R17 mean?","a":"225 is the section width in mm, 45 the sidewall height as a percentage of the width, R radial construction and 17 the rim diameter in inches. A trailing 91W is the load index and speed rating."},{"q":"How much can the diameter change?","a":"Most manufacturers and fitters recommend staying within ±3 % of the original diameter so the speedometer, ABS and stability control remain accurate and the tire clears the wheel arch."}],"tags":["tire size calculator","tyre size","tire diameter","tire code","plus sizing","speedometer error"],"related":[{"calculator_id":"gear-ratio-speed","reason":"Road speed from rpm, gearing and this tire diameter."},{"calculator_id":"circle","reason":"Circumference and area of the tire circle."},{"calculator_id":"speed","reason":"Convert the corrected speed between km/h and mph."}],"links":{"html":"https://tttkmbb.com/engineering/tire-size","markdown":"https://tttkmbb.com/engineering/tire-size.md","json":"https://tttkmbb.com/engineering/tire-size.json","api":"https://tttkmbb.com/api/v1/calculate/tire-size","schema":"https://tttkmbb.com/api/v1/calculators/tire-size","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T01:45:05Z"}