Methodology and sources

Everything on this site is one of two things: arithmetic on a size code you typed, or a value copied from a cited page on a stated date. This page lists both, plus the one assumption that is ours and the reason there is no vehicle database here.

1. Constants

2. Tire geometry

A metric size gives section width in millimetres, aspect ratio as a percentage of that width, and rim diameter in inches. Everything else follows:

sidewall = width × aspect ÷ 100

overall diameter = rim × 25.4 + 2 × sidewall

circumference = π × overall diameter

revolutions per mile = 1,609,344 ÷ circumference    revolutions per km = 1,000,000 ÷ circumference

A flotation size such as 33x12.50R17 states overall diameter and section width in inches, so those are converted at 25.4 and the sidewall is derived as (diameter − rim) ÷ 2. The aspect ratio shown for a flotation size is derived from those figures, not read off the sidewall.

These are nominal dimensions calculated from the size designation. They are not measurements of a mounted tire: a real tire's diameter varies with brand, construction, measuring rim width, inflation pressure and load, which is why every output carries one decimal place and no claim of precision beyond it. Nothing on this site was measured on a vehicle.

Comparison and speedometer error

For a current size A and a candidate size B:

diameter difference % = (D_B ÷ D_A − 1) × 100    ride height change = (D_B − D_A) ÷ 2

A speedometer driven from wheel or driveshaft rotation counts revolutions and multiplies by the circumference it was calibrated for. Fit a tire of different diameter and the needle is wrong by the diameter ratio:

actual speed = indicated speed × (D_B ÷ D_A)

The tool prints that at 30, 45, 60 and 75 mph, or 50, 80, 100 and 120 km/h in metric mode. This is the pure geometric error. It does not include the deliberate bias many vehicles carry from the factory, where the speedometer reads slightly high, and it does not model tire growth at speed. Both effects are real and neither is published per vehicle in any source we can cite, so neither is in the arithmetic.

The ±3% convention

The tire tool flags a diameter change greater than ±3% as outside the convention most fitters use. This is a widely repeated trade rule of thumb, not a standard, not a regulation and not a manufacturer specification. It is in the tool because it is the threshold readers will have been quoted elsewhere, and it is labelled as a convention everywhere it appears. A change inside ±3% is not automatically safe, and a change outside it is not automatically unsafe; the questions it should trigger are clearance, load rating, ABS and stability-control calibration, and gearing.

3. Wheel geometry

Offset (ET) is the distance from the hub mounting face to the wheel's centreline, positive when the mounting face sits toward the street side. Backspacing is the distance from that same mounting face to the inboard rim edge. The centreline sits at half the overall width, so:

overall width = nominal width + flange allowance

backspacing = overall width ÷ 2 + offset    frontspacing = overall width ÷ 2 − offset

offset = (backspacing − overall width ÷ 2) × 25.4

Changing from wheel A to wheel B, the two edge movements are reported separately:

inboard movement = BS_B − BS_A    outboard movement = FS_B − FS_A

4. The flange allowance, our one assumption

Nominal wheel width is measured between the bead seats. Backspacing is measured to the rim edge, which is outside the bead seat by the width of the flange at each end. Converting between offset and backspacing therefore needs a figure for the two flanges together. We use 1.00 inch, the usual trade convention, and the input is editable so that a wheel drawing with the real figure can override it.

This matters because it shifts the result: at 1.00 inch of allowance, an 8.0-inch wheel at ET35 has 5.88 inches of backspacing; at 0.5 inch it would compute as 5.63 inches. Any site that converts offset to backspacing is making this assumption, whether or not it says so. Ours is stated, labelled and adjustable. The underlying definitions of rim width, bolt circle, offset and backspacing follow Modern Tire Dealer, "Wheel Dimensions", retrieved 2026-10-16.

5. Bolt circle geometry

The bolt circle diameter, or PCD, is the diameter of the circle through the stud centres. For n studs on a circle of diameter PCD, the straight-line distance between two adjacent stud centres is:

chord = PCD × sin(180° ÷ n)

That identity is what makes an even-lug pattern measurable directly across the hub and an odd-lug pattern not. Two patterns of the same stud count differ at every hole by half the PCD difference, measured radially. The measuring methods by lug count, and the consequences of a near-match, are set out in the bolt pattern guide and the 5x114.3 vs 5x115 post. No bolt pattern on this site is attributed to any vehicle.

6. Sidewall reference tables

The load index, speed symbol and service-type tables in what a tire size means are copied from the pages below on 2026-10-16 and published as /data/markings.json with this provenance embedded. Values outside a source's printed range are shown as not available rather than extrapolated.

7. Why there are no vehicle fitment pages

The obvious version of this site is a page per vehicle: bolt pattern, centre bore, factory tire sizes, offset range. We have not built it, and the reason is not technical.

Vehicle fitment data of that kind is a licensed commercial product. The Wheel-Size developer portal sells API access covering OE and aftermarket tire sizes, rim dimensions, offset and bolt patterns. Its API terms of usage, read on 2026-10-16, state that the API cannot be resold under any circumstances, that a subscriber must not supply or redistribute fitment data to other businesses through an intermediary service, that data can be saved locally for SEO purposes within the plan's rate limits across a named set of cataloguing endpoints, and that access may be suspended or terminated for a violation of the terms. In other words a licence is obtainable and its scope is specific, which is exactly why the scope has to be agreed in writing before a public, indexed vehicle database is built on it rather than after. Scraping a competitor's fitment pages is not an alternative; it is the same data without the licence.

The remaining option would be to generate plausible-looking vehicle specifications, and that is the one thing we will not do. A bolt pattern is a safety number. A fabricated pattern that is 0.7 mm out looks like a match on the car and is not one. So this site states plainly: it holds no vehicle data at all. If a licence covering indexed display and caching is obtained, vehicle pages would appear at a new path, sourced and dated, and this section would say so. Until then the authorities for your vehicle are the Tire and Loading Information label on the driver's side door edge and the owner's manual.

8. What the tools cannot do

• They do not know your clearances. Fender, strut, liner and brake clearance is a measurement on your own car at full compression and full lock, and it differs side to side.
• They do not check load capacity. The load index on the tire has to cover the axle load on the vehicle's label; the tools do no such comparison.
• They do not know whether your manufacturer approves a size or wheel change, or how your ABS and stability control are calibrated.
• They report nominal dimensions from the size code, not measured dimensions of a specific tire on a specific rim at a specific pressure.

9. Corrections

If a figure here does not follow from the formulas above, or a table value does not match its cited source, that is a bug. The data file carries a version (2026-10-16) and the source table carries its retrieval date, so a discrepancy can be traced to either the arithmetic in src/js/fitment.js or the fetch. Both the build and the browser run that same module, so a page and the tool cannot disagree.