Tire Size Calculator (Spec / Diameter / Speedometer Error)
Enter or paste a P-metric tire spec (e.g. 225/65R17) to see sidewall height, overall diameter, circumference, revolutions per km / mile, and the speedometer error against any reference size — essential before swapping tires or going up a wheel size (plus-one, plus-two).
Tire spec
Reference tire (for comparison)
Enter valid values: width 100–500 mm, aspect 20–100%, wheel 8–30 inches.
Computed dimensions
Sidewall height
146.3 mm
5.76 in
Overall diameter
724.3 mm
28.52 in
Tire circumference
2275.5 mm
89.59 in
Revolutions / km
439.5
Revolutions / mile
707.3
Compared to reference
+2.30%
larger than reference
Speedo reads 100 km/h → actual ≈
102.3 km/h
Applies to P-metric / passenger-car tires only — flotation sizes such as LT 31×10.5R15 are not covered. Real-world rolling radius is typically 1–2% smaller than theoretical due to inflation pressure, load and wear.
Formula
sidewall = width(mm) × aspect ÷ 100 | diameter(mm) = wheel(in) × 25.4 + 2 × sidewall | circumference = π × diameter | speedo error ≈ candidate diameter ÷ reference diameter − 1
- · P-metric notation: width(mm)/aspect(%)R wheel(in). e.g. 225/65R17 — 225 mm section width, sidewall = 225 × 65% = 146.25 mm, 17-inch wheel.
- · Speedometer (and odometer) is calibrated against the original tire circumference. A larger tire makes it under-read (shows 100 km/h while you are faster); a smaller tire over-reads. Rule of thumb: keep overall diameter within ±3 % of stock.
- · Plus-one upgrade (wheel +1″): aspect ratio typically drops by 10 to keep diameter the same. Example: 225/65R17 → 225/55R18 differs by only ~0.4 % in diameter.
- · Only metric / P-metric passenger sizes are covered here. Light-truck "flotation" sizes such as LT 31×10.5R15 use overall-diameter × width R wheel format and must be converted separately.
- · Theoretical vs rolling radius: a loaded, inflated tire typically rolls 1–2 % smaller than the geometric value due to load deflection, pressure and wear, which affects odometer accuracy.
- · Source: Tire and Rim Association Year Book, ETRTO Standards Manual, and SAE J1858 — the industry-standard dimensional definitions.
Frequently asked
What do the numbers in 225/65R17 actually mean?
225 is the section width in millimetres. 65 is the aspect ratio — sidewall height as a percentage of width (225 × 0.65 = 146.25 mm). R denotes radial-ply construction. 17 is the wheel diameter in inches. Plugging into "diameter = wheel × 25.4 + 2 × sidewall" gives 17 × 25.4 + 2 × 146.25 ≈ 724.3 mm.
How much diameter mismatch is OK before the speedometer is off?
Most factories calibrate within roughly ±3 %, which keeps speedometer error under ~3 km/h at 100 km/h. Going from 225/65R17 to 245/65R17 increases overall diameter by ~3.6 %, so an indicated 100 km/h is actually ~103.6 km/h — already at the edge of the tolerance. Mismatches beyond ~5 % can affect ABS, ESP and TPMS calibration and should be avoided.
How does plus-sizing work — by how much must the aspect ratio drop?
The principle is to keep overall diameter near stock. Going up one wheel size, aspect ratio typically drops by ~10 (and width often increases slightly to keep load capacity). A classic plus-sizing chain: 195/65R15 → 205/55R16 → 215/45R17 → 225/40R18. Type both sizes into this tool, compare diameter / circumference / speedo, and pick the candidate closest to stock.
Why do tires of the same spec from different brands measure slightly differently?
ETRTO allows a small manufacturing tolerance on top of the published design section, and tread depth, compound and mould era all add small variations — so a 225/65R17 from Michelin, Yokohama and Bridgestone can differ by 5–10 mm in measured diameter. This tool reports the theoretical geometric value; for a specific brand check that maker's spec sheet "Overall Diameter" column.
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