How every figure here is produced

The geometry

A metric tyre size is a specification that defines its own dimensions. Sidewall height is the aspect ratio applied to the section width; overall diameter is the rim plus two sidewalls; circumference follows from diameter, and revolutions per kilometre follow from circumference.

sidewall (mm)          = width × aspect ÷ 100
overall diameter (mm)  = rim × 25.4 + 2 × sidewall
circumference (mm)     = π × overall diameter
revolutions per km     = 1,000,000 ÷ circumference
true speed             = indicated × (new diameter ÷ old diameter)

Nothing above is measured, sampled, averaged or dated, which is why this is the one site in the family with no data vintage on its figures. The source line reads ETRTO STANDARD instead of a month, and that is not a dodge — it is the accurate statement.

What a tyre actually measures, and what this does not model

Overall diameter as computed here is the unloaded, nominal figure. A real tyre on a real car is slightly shorter under load, and its rolling circumference is shorter again because the contact patch is flat while the rest of the tyre is round. Manufacturers publish a separate rolling circumference that accounts for this and it typically runs 2–3% below π × D.

This matters for one reason and no others: the comparisons here are ratios between two tyres, and the deflection correction very nearly cancels in a ratio. So the percentage figures — the ones the verdicts and the speedometer errors are built on — are robust. The absolute circumference is the nominal one, and this paragraph is where that is stated rather than buried.

The ±3% rule, and what it is not

±3% of rolling diameter is an industry convention. It is not a law, nothing legal happens at 3.01%, and no regulator anywhere publishes it. It is meaningful because vehicle manufacturers validate the speedometer, ABS and stability-control calibration inside a band of that order, so staying within it keeps you inside the envelope the car was actually developed against.

The three verdicts this site prints are within tolerance (±3%), marginal (3–5%), outside tolerance (over 5%). Two states would have been easier to design and would have been a lie: the space between "fine" and "don't" is real, and most interesting fitments live in it.

The speedometer rule

UN ECE Regulation 39 governs speedometers in New Zealand, Australia, the United Kingdom and the European Union. It sets an asymmetric requirement: the indicated speed may never be below true speed, and may exceed it by at most 10% + 4 km/h. The United States has no federal accuracy standard for passenger-car speedometers — FMVSS 101 requires the instrument, not its accuracy.

This asymmetry is why the direction of a tyre change matters more than its size. Taller tyres push the reading down, into the half of the rule with zero tolerance.

Which sizes get a page, and which get indexed

The size universe is generated from the standard's own increments — section widths in 10 mm steps, aspect ratios in 5-point steps, rim diameters in whole inches — and then filtered twice.

Stage one, physical. Overall diameter between 530 mm and 860 mm, sidewall between 60 mm and 215 mm. Outside that it is a kart tyre or a tractor tyre. 1,945 designations survive.

Stage two, conventional. The combinations the road-car market actually builds. 1,220 of the 1,945 pass, and they are the ones offered to search engines. The remaining 725 still get a page — the arithmetic is still correct and someone may still want it — but they are served noindex,follow, are marked as uncommon in every index, and are never presented as things you can buy. The rules, in full:

• A 30- or 25-series sidewall is only built for 17-inch rims and larger.
• A 40-series is only built for 16-inch rims and larger.
• A 70-series or taller is not built above a 17-inch rim.
• A 65-series is not built above a 19-inch rim.
• 285 mm and wider needs at least a 17-inch rim to carry it.
• 165 mm and narrower is not built above a 16-inch rim.
• Very wide and very tall together (275 mm+ on a 65-series or more) is a farm tyre, not a road tyre.
• Very narrow and very low together (185 mm or less on a 35-series or less) is not made.
• 305 mm and wider is not built on a 55-series or taller.
• 175 mm and narrower is not built on a 40-series or lower.

Comparison pages exist for 816 pairs, chosen by the same logic: a pair earns a page when a driver could plausibly be choosing between the two — same rim within 6% on diameter, or one rim size up within 3%, which is what plus-sizing is. Each pair has exactly one URL whichever way it is phrased.

What this site does not know

It does not know whether a size is currently manufactured, what it costs, who makes it, or whether it will fit your particular car. Rim width ranges, arch clearance, suspension travel, wheel offset, load index and speed symbol are all decisive and none of them are in a size string. The tyre placard in your driver's door aperture is the authority on your vehicle, and it outranks this site.

There was briefly a function here that estimated the load index a size is "typically" built to. It returned 1,000 kg for a size that is really 615 kg, and it was deleted rather than tuned: an unsourced number on a page about whether something is safe to fit has no business existing. The load index appears on this site only as the lookup table it actually is.

Corrections

If a figure here is wrong, it is wrong by arithmetic and can be demonstrated in one line. Send it and it will be fixed.

SOURCE·Metric size geometry + UN ECE R39 speedometer tolerance·REVIEWED ETRTO STANDARD