# Tire Size Calculator

> 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).

- Calculator id: `tire-size` · Category: Engineering & Automotive (`engineering`) · Tool name: `calculate_tire_size`
- Canonical page: https://tttkmbb.com/engineering/tire-size · This document: https://tttkmbb.com/engineering/tire-size.md · JSON definition: https://tttkmbb.com/engineering/tire-size.json

## Purpose

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).

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `tire_code` | string |  | required | 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). |
| `compare_tire_code` | string |  | optional | Optional second (new) tire size to compare against tire_code (the original), e.g. 235/40R18. |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `width_mm` | number | mm | Nominal section width. |
| `aspect_ratio_percent` | number | % | Sidewall height as a percentage of the width. |
| `rim_diameter_in` | number | in | Wheel diameter in inches. |
| `sidewall_height_mm` | number | mm | width × aspect ratio / 100. |
| `overall_diameter_mm` | number | mm | 2 × sidewall + rim × 25.4. |
| `overall_diameter_in` | number | in | Overall diameter in inches. |
| `circumference_mm` | number | mm | π × overall diameter. |
| `revolutions_per_km` | number | rev/km | 1,000,000 / circumference_mm. |
| `revolutions_per_mile` | number | rev/mile | 1,609,344 / circumference_mm. |
| `compare_overall_diameter_mm` | number | mm | Overall diameter of compare_tire_code (only when given). |
| `diameter_difference_mm` | number | mm | compare − original (only when compare_tire_code is given). |
| `diameter_difference_percent` | number | % | (compare − original) / original × 100; keep within about ±3 %. |
| `actual_speed_at_indicated_100` | number | km/h or mph | With the compared tire fitted, true speed when the speedometer (calibrated for the original) reads 100 (only when compare_tire_code is given). |
| `size_change_assessment` | string |  | Whether the diameter change stays within the common ±3 % guideline (only when compare_tire_code is given). |

## 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`

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.

## Data Sources

- Wikipedia – Tire code — https://en.wikipedia.org/wiki/Tire_code (reference, retrieved 2026-09-24)
- ETRTO – European Tyre and Rim Technical Organisation standards manual — https://www.etrto.org/ (standard, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/tire-size?tire_code=…`
- `POST https://tttkmbb.com/api/v1/calculate/tire-size` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/tire-size · OpenAPI operationId `calculate_tire_size` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "tire-size", "inputs": {…}}`

## Example

- 225/45R17: inputs `{"tire_code":"225/45R17"}` → `{"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}`
- 205/55R16 replaced by 225/45R17: inputs `{"tire_code":"205/55R16","compare_tire_code":"225/45R17"}` → `{"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"}`

```
GET https://tttkmbb.com/api/v1/calculate/tire-size?tire_code=225%2F45R17
```

## Limitations

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). 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. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**What do the numbers in 225/45R17 mean?**

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.

**How much can the diameter change?**

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.

## Related

- [Gear Ratio and Speed Calculator](https://tttkmbb.com/engineering/gear-ratio-speed.md) — Road speed from rpm, gearing and this tire diameter.
- [Circle Calculator](https://tttkmbb.com/geometry/circle.md) — Circumference and area of the tire circle.
- [Speed Converter](https://tttkmbb.com/conversion/speed.md) — Convert the corrected speed between km/h and mph.