How these numbers are calculated

Every dimension on this site is computed from the size code by one library at build time. Nothing is stored, so nothing can drift; the same functions serve the reference pages and the calculators. This page states what those functions do.

Reading a size code

Two notations are in use, and the site parses both.

Metric, written 285/70R17. The first number is nominal section width in millimetres. The second is the aspect ratio: the sidewall height as a percentage of that width. The letter is construction — R for radial, D (or a plain hyphen) for diagonal bias, B for bias-belted, and ZR where a Z speed rating is carried in the size position. The last number is wheel diameter in inches. A leading P, LT, ST or T is the service type and does not change the geometry.

Flotation, or imperial, written 33x12.50R17. The first number is nominal overall diameter in inches, the second is section width in inches, the last is wheel diameter in inches. There is no aspect ratio printed, so the site derives it.

The formulas

sidewall height  = section width x aspect ratio / 100
overall diameter = wheel diameter + 2 x sidewall height
circumference    = pi x overall diameter
revs per mile    = 63360 / circumference in inches
aspect ratio     = sidewall height / section width x 100     (derived, flotation sizes)

Speedometer error is the ratio of two overall diameters:

actual speed = indicated speed x new diameter / original diameter

A speedometer counts wheel revolutions. Change the distance covered per revolution and the reading scales by exactly that proportion — there is no other term. Modern vehicles take the signal from an anti-lock brake sensor rather than a cable, which changes nothing: the pulse count per wheel revolution is fixed either way.

Engine speed at a road speed is:

rpm = mph x revs per mile x transmission ratio x axle ratio / 60

Most workshops use a shorter version, mph x ratio x 336 / diameter, and the two agree exactly: the 336 is just what you get when the inches-per-mile figure and the seconds-per-minute conversion are collapsed into one number, which works because a circumference is a fixed multiple of a diameter. Going the long way round — through revolutions per mile — is deliberate here, because it is the same quantity the size pages print, so a gearing answer and a size answer cannot come from two different places and disagree.

To put engine speed back where it started once a taller or shorter tire is on the vehicle:

new ratio = old ratio x old revs per mile / new revs per mile

which is the diameter ratio expressed the other way round.

The tables

Load index and speed symbol come from the standardised published series that every manufacturer reproduces identically. The kilogram column is the standard series; the pound column is converted from it in code rather than typed, so the two cannot disagree. Speed symbols are listed in chart order, which is why H appears between U and V rather than in alphabetical position — it was standardised before the rest of the series and kept its place.

Where the arithmetic stops matching the tire

This is the honest part, and it matters more than the formulas.

Section width is specified at a nominal measuring rim. Mount the same tire on a rim half an inch wider and it measures roughly 0.2 in wider. The number in the size code is a specification, not a measurement of your tire on your wheel.

Overall diameter is a free diameter. A loaded tire deflects, so its rolling radius is smaller than half its free diameter. Manufacturers measure their revolutions-per-mile on a loaded tire and this site derives it from the size code, which is why a published figure and the one here will not match exactly — expect the measured one to be the higher of the two by a small margin. Where a manufacturer publishes a measured figure for a specific tire, use it in preference to this one.

Nominal sizes are nominal. Two tires with the same code from two manufacturers can differ by several tenths of an inch in measured diameter, and often do. Aggressive tread patterns commonly measure under their stated size.

Flotation sizes round hard. A tire sold as a 35 is often nearer 34.5 in when measured, because the marked diameter is a class, not a specification.

None of this makes the arithmetic wrong. It makes it what it is: an exact answer about the size code, which is a good estimate of the tire and not a measurement of it. Where a decision turns on a tenth of an inch, measure the actual tire.

Platform figures

Vehicle pages record factory tire sizes, wheel diameters and, where they could be confirmed, axle ratios. Each record states what it was read from and when it was last checked. Where a figure could not be confirmed, the field is omitted rather than filled — an invented ratio produces wrong engine-speed arithmetic on a page that looks authoritative, which is worse than a gap.

Your vehicle's own tire and loading information placard, on the driver's door jamb, is the authoritative record for your specific vehicle. Read it before trusting any table, including the ones here.