Tire Size Calculator

A tire size calculator reads a metric tire code such as 225/45R17 and works out the tire's nominal dimensions: sidewall height, overall diameter, circumference, and revolutions per mile. Given two sizes, it also shows how much the diameter changes and how far the speedometer will be off after the swap.

How to read a tire size

Metric tire designations follow ISO 4000-1 and the ETRTO Standards Manual. The code 225/45R17 breaks down as:

225 is the section width in millimetres: how wide the tire is from sidewall to sidewall.
45 is the aspect ratio in percent: the sidewall height is 45% of the section width.
R is the construction code. R means radial. The calculator also accepts ZR (high-speed radial), D (diagonal), B (belted bias), and a plain hyphen.
17 is the rim diameter in inches. Half-inch rims such as 16.5 are accepted.

A P (passenger) or LT (light truck) prefix may appear before the width. A load index and speed rating, such as "94W" in "225/45R17 94W", may follow the size. The calculator strips this suffix; it does not affect the geometry.

The calculator accepts widths from 100 to 500 mm, aspect ratios from 20 to 95 percent, and rim diameters from 8 to 30 inches. Flotation sizes written in inches, such as 31x10.50R15, are not supported and produce an error rather than a guess.

Tire size formulas

The geometry follows from the designation itself:

Sidewall height = section width × aspect ratio ÷ 100
Overall diameter = rim diameter × 25.4 + 2 × sidewall height
Circumference = π × diameter
Revolutions per kilometre = 1,000,000 ÷ circumference (in mm)
Revolutions per mile = 1,609,344 ÷ circumference (in mm)

The rim diameter is multiplied by 25.4 because one inch is exactly 25.4 mm, and the sidewall is counted twice because it sits both above and below the rim. One mile is exactly 1,609.344 m. Both conversion factors were fixed by the 1959 International Yard and Pound Agreement and are listed in NIST Special Publication 811.

When two sizes A and B are entered, the comparison is a simple ratio of their diameters:

Diameter difference (%) = (diameter B ÷ diameter A − 1) × 100
Speedometer error (%) = the same ratio, because the speedometer counts wheel revolutions
Actual speed = indicated speed × (diameter B ÷ diameter A)

Worked example: 225/45R17 vs 225/40R18

Tire A is 225/45R17, the original size. Tire B is 225/40R18, a common "plus one" upgrade to a larger rim.

Value	225/45R17 (A)	225/40R18 (B)
Sidewall height	225 × 45 ÷ 100 = 101.25 mm	225 × 40 ÷ 100 = 90.00 mm
Overall diameter	17 × 25.4 + 2 × 101.25 = 634.30 mm	18 × 25.4 + 2 × 90.00 = 637.20 mm
Circumference	π × 634.30 = 1,992.71 mm	π × 637.20 = 2,001.82 mm
Revolutions per mile	1,609,344 ÷ 1,992.71 = 807.6	1,609,344 ÷ 2,001.82 = 803.9

The new tire is +2.90 mm larger in diameter, a change of +0.46%. The circumference grows by +9.11 mm, and the tire turns 3.7 fewer times per mile. At an indicated 60 mph, the true speed on the new tires is about 60.3 mph. A change this small is well inside the common 3% replacement guideline.

Speedometer and odometer error

A car's speedometer and odometer do not measure speed or distance directly. They count wheel revolutions and multiply by the circumference the factory assumed. Fitting a tire with a different diameter changes the real distance per revolution, but the instruments keep using the old assumption.

A larger tire covers more ground per turn, so the speedometer reads low: the car is going faster than shown, and the odometer records fewer miles than were driven. A smaller tire does the opposite. The calculator reports this as a signed percentage. A positive speedometer error means the true speed is higher than the indicated speed.

This is the geometric error from the size change alone. Real speedometers are deliberately biased: UNECE Regulation No. 39 requires that a speedometer never read below the true speed, and allows it to read up to 10% plus 4 km/h above it. A car that showed 3% high on the original tires will still show roughly 3% high, shifted by the geometric change.

The 3% guideline

The calculator shows a warning when the diameter changes by more than 3%. This threshold is a widely used fitment rule of thumb, not a regulation. Beyond it, the speedometer error becomes noticeable, gearing and fuel-economy readings shift, and the tire may contact the wheel arch or suspension. The warning is informational; clearance and load capacity still have to be checked against the vehicle and the tire manufacturer's documentation.

Nominal versus real dimensions

All results are nominal: they come from the numbers printed on the sidewall, following the ETRTO convention that the sidewall height equals the section width times the aspect ratio. A real mounted tire differs from this ideal. Actual dimensions vary with the manufacturer's mould, the rim width, inflation pressure, load, and tread wear, and a loaded rolling tire covers slightly less ground per revolution than its unloaded circumference suggests. Manufacturer data sheets list measured values that can differ from the nominal figures by a percent or so. Since both tires in a comparison are computed the same way, these effects largely cancel in the percentage results.

Frequently asked questions

What do the numbers in 225/45R17 mean?

225 is the tire's width in millimetres, 45 means the sidewall is 45% as tall as the tire is wide, R means radial construction, and 17 is the rim diameter in inches.

Does a bigger tire make the speedometer read high or low?

Low. A larger diameter covers more ground per revolution, so the true speed is higher than the displayed speed. The odometer also under-counts distance.

How much bigger can a replacement tire be?

A common guideline is to stay within 3% of the original overall diameter. The calculator flags larger changes. It is a rule of thumb; body clearance and the vehicle documentation decide what actually fits.

Why do these numbers differ from the manufacturer's data sheet?

The calculator uses nominal dimensions derived from the size code. Manufacturers publish measured dimensions for a specific tire on a specific rim, which can differ slightly.

Can it calculate inch-based sizes like 31x10.50R15?

No. Flotation (inch) sizes use a different format, in which the first number is already the overall diameter. The calculator only parses metric codes and reports an error for anything else.

Do the load index and speed rating change the result?

No. A suffix such as 94W describes the tire's load and speed capability, not its size. The calculator ignores it when computing geometry.

Tire Size Calculator: Diameter & Speedometer Error