Formulas & methodology

Metric sizes (e.g. 265/70R17)

sidewall (mm)  = width (mm) × aspect ratio ÷ 100
diameter (in)  = rim (in) + 2 × sidewall (mm) ÷ 25.4
width (in)     = width (mm) ÷ 25.4

Inch / flotation sizes (e.g. 33x12.50R17)

diameter (in)  = first number
width (in)     = second number
sidewall (in)  = (diameter − rim) ÷ 2
aspect ratio   = sidewall ÷ width × 100

Derived values

circumference      = π × diameter
revolutions/mile   = 63,360 ÷ circumference (in)
revolutions/km     = 1,000,000 ÷ circumference (mm)
speedometer factor = new diameter ÷ original diameter
true speed         = indicated speed × speedometer factor
clearance change   = (new diameter − original diameter) ÷ 2
engine RPM         = mph × axle ratio × gear ratio × 336.14 ÷ diameter (in)
regear ratio       = axle ratio × new diameter ÷ original diameter

The constant 336.14 is 63,360 inches per mile ÷ (60 minutes × π).

Equivalent and plus-size searches

We enumerate standard metric combinations — widths from 155 to 355 mm in 10 mm steps and aspect ratios from 25 to 85 in steps of 5 — on the chosen wheel, compute each diameter, and keep those within the chosen tolerance (±3% by default). The ±3% figure is a rule of thumb often quoted in the tire trade — not a legal limit and not a fitment approval; your vehicle maker's specification and local regulations take precedence. The search does not check whether a tire is actually manufactured in that size.

Reference tables and sources

The load index and speed symbol tables reproduce the standardized values in the tire standards listed below; pounds are converted from kilograms (×2.20462), so a few values differ by 1–3 lb from US charts that round to 5 lb. Our automated tests cross-check published reference points (for example load index 94 = 670 kg / 1,477 lb).

• ISO 4000-1 (passenger car tyres and rims — designation, dimensions, load capacity) and ISO 4209-1 (truck and bus tyres).
• ETRTO Standards Manual (European Tyre and Rim Technical Organisation) and The Tire and Rim Association Year Book (USA).
• UN Regulations No. 30 and No. 54 (tyre approval, load and speed symbols) and No. 39 (speedometers).
• US 49 CFR 574.5 (tire identification number, including the date code) and FMVSS No. 139 (light-vehicle tires).

Values entered by hand

• The load index and speed symbol tables (from the standards above).
• The selection of sizes and comparisons that have their own pages, and the sizes in the tire size chart.
• The reference list of axle ratios (3.21–5.38) used to suggest nearby ratios, the 3.73 default axle ratio, and example gear ratios.
• The ±3% rule-of-thumb threshold and the rim-width rule of thumb mentioned below.

Known limits

• All dimensions are nominal: a new tire, unloaded, on its measuring rim. A specific model can be a few tenths of an inch taller or shorter; published "revolutions per mile" from manufacturers are measured under load and are a little higher than ours.
• Section width changes with wheel width (a common rule of thumb is about 5 mm per half inch of rim width).
• We do not provide vehicle fitment, tire pressure or load-carrying advice. Use your vehicle placard, owner's manual and a qualified installer.

Quality control

The calculation engine has an automated test suite that checks parsing of every supported size format, reference diameters published elsewhere (for example 245/75R16 = 30.47 in and 265/75R16 = 31.65 in), internal consistency (circumference × revolutions = one mile), the load index table and the date-code decoder. The site is rebuilt and the tests run on every change. Found an error? Please tell us on the corrections page.