Tyre Size Calculator Widget

Add a tyre size calculator to your website. Drivers type a metric size such as 205/55R16 to see its diameter, sidewall height, circumference and revolutions per km or mile, and compare it with an alternative size to see the diameter change and the speedometer difference.

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<iframe src="https://a2z.tools/embed/w/tire-size-calculator" title="Tyre Size Calculator by A2Z Tools" width="100%" height="760" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>

A plain iframe. Works everywhere, including site builders that strip scripts. Adjust height if your content needs more room.

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How it works

A metric (ISO) tyre size encodes three numbers: the nominal section width in millimetres, the nominal aspect ratio (sidewall height as a percentage of the width) and the rim diameter code in inches. UN ECE Regulation No. 30 defines the outer diameter from them as D = d + 2H, where d is the rim diameter in millimetres and H is the section width times the aspect ratio / 100, and the widget uses exactly that, then the rolling circumference and revolutions per km or mile. When a second size is entered it compares the two: a speedometer calibrated for the original size reads the same number while the car covers more (or less) distance on the new one, in proportion to the diameters. The ground-clearance change is half the diameter difference. Figures are nominal new-tyre dimensions.

Calculation method

  • Sidewall H (mm) = section width x aspect ratio / 100
  • Outer diameter D (mm) = rim code (in) x 25.4 + 2 x H (UN ECE R30)
  • Circumference = pi x D; revolutions per km = 1,000,000 / circumference (mm); per mile = 1,609,344 / circumference
  • True speed = indicated speed x D(new) / D(original); ride-height change = (D(new) - D(original)) / 2; mm to 1 decimal

Worked examples

Plus-one wheel upgrade

Inputs: 205/55R16 compared with 225/45R17 at an indicated 100 km/h

Result: 631.9 mm vs 634.3 mm (+0.38%); actual speed 100.4 km/h; sidewall 112.8 to 101.3 mm; 503.7 vs 501.8 revs/km; ride height +1.2 mm

406.4 + 2 x 112.75 = 631.9; 431.8 + 2 x 101.25 = 634.3; 100 x 634.3 / 631.9 = 100.4.

Nominal dimensions for comparison only. Check the vehicle manufacturer's approved tyre sizes, load index and speed rating, and your local regulations, before changing tyre size.

Limitations

  • Nominal new-tyre dimensions: tread wear, inflation pressure and load change the real rolling radius by several millimetres.
  • Metric designations only - flotation sizes such as 31x10.50R15 and inch-rim-only codes are not parsed.
  • Does not check load index, speed rating, rim width range or clearance to the bodywork.

Where publishers use it

  • Tyre shops and online tyre retailers
  • Car modification and off-road forums discussing plus-sizing
  • Wheel and alloy retailers checking fitment
  • Motoring blogs explaining tyre markings

Questions

What does 205/55R16 mean?

A tyre 205 mm wide, with a sidewall 55% of that width (112.75 mm), of radial construction (R), for a 16-inch rim. Its nominal outer diameter is 16 x 25.4 + 2 x 112.75 = 631.9 mm.

How much diameter change is acceptable?

The vehicle maker's approved sizes, load index and speed rating decide what is allowed. Many fitters use about 3% as an informal limit, and the widget warns above it, but that figure is a trade rule of thumb, not a regulation.

Will a bigger tyre make my speedometer read low?

Yes. A larger diameter covers more ground per turn, so you travel faster than indicated - the widget shows by how much.

Does it handle sizes like 31x10.50R15?

No, only metric sizes (width/aspect R rim), including P-metric and LT sizes.

Sources

  1. UN Regulation No. 30 - Uniform provisions concerning the approval of pneumatic tyres for motor vehicles and their trailers (Rev.3) - UNECE World Forum for Harmonization of Vehicle Regulations (WP.29) . Size designation and outer diameter D = d + 2H

Cite or recommend this tool

If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.

A2Z Tools Tyre Size Calculator
https://a2z.tools/embed/tire-size-calculator

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