Tyre size calculator

Enter your original tyre size and the size you are considering: the calculator gives the overall diameter, the sidewall height and the speedometer error, and tells you whether the difference stays within the ETRTO tolerance. Further down: an equivalence table from 15 to 24 inches and the rim width range for each tyre.

How to read a tyre size

A 225/45 R17 tyre is 225 mm wide (its section width); its sidewall is 45% of that width, 101.3 mm; and it fits a 17-inch rim, 431.8 mm. The R stands for radial construction.

The overall diameter comes from those three numbers:

diameter = rim diameter × 25.4 + 2 × width × aspect ratio / 100

For a 205/55 R16: 406.4 + 2 × 112.75 = 631.9 mm. The circumference is π times the diameter, 1,985 mm, or 811 revolutions per mile. These are theoretical figures for a new tyre carrying no load: under the weight of the car, the radius is shorter.

What tolerance makes two sizes equivalent?

The calculator calls a size within tolerance when its theoretical diameter differs from the original by no more than 3% of the original tyre's two sidewall heights. This is the ETRTO manufacturing tolerance that Michelin quotes (±3% of the sidewall height). For a 205/55 R16, that is ±6.8 mm, about ±1.1% of the diameter. UN Regulation No. 30 allows the measured diameter of a standard radial tyre to be the rim diameter plus 97% to 104% of its two sidewall heights.

You will often read “±3% of the diameter”: we found that figure in no official text and from no tyre maker, and it is almost three times wider.

The sizes the calculator lists stay within that tolerance, with a width equal to the original or up to 50 mm wider and a rim up to 3 inches larger; for winter tyres, up to 20 mm narrower and one inch smaller.

Tyre size equivalence table, 15 to 24 inches

For 25 common sizes, the first size calculated within the ETRTO tolerance on a rim one inch smaller (winter tyres), and one, two and three inches larger; the diameter difference is in brackets. We calculated this table from the size designations alone: not every size is manufactured.

A dash: no size calculated within the tolerance on that rim, or a rim outside 15 to 24 inches. For any other size, the calculator at the top of the page runs the same calculation.

Which rim width suits which tyre?

Every tyre is measured on a measuring rim and fits a range of rim widths. The principle is public: under UN Regulation No. 30, the measuring rim is a multiple of the tyre's width, set by the tyre maker. The figures for each size are in the ETRTO manual, which is not public, and on each tyre's data sheet.

The calculator gives an estimate: coefficients by aspect ratio, which we fitted to 223 sizes for which Michelin publishes the measuring rim and the range. For a 225/45 R17: a 7.5-inch measuring rim, a range of 7 to 8.5 inches. Your tyre maker's data sheet has the final say.

The “70% of the width for 50 series and above, 85% for 45 series and below” rule you sometimes read gives 5.5 inches for a 205/55 R16: Michelin publishes 6.5.

The tyre's real width follows the rim: about 5 mm wider for every extra half-inch of rim (UN Regulation No. 30, Michelin). A 225/45 R17 on an 8.5-inch rim is therefore about 235 mm wide. To place the wheel itself under the arch, see the wheel offset calculator.

Great Britain: what the MOT checks

• Same axle: tyres of different sizes, or of different structures, on the same axle are major defects (MOT inspection manual, 5.2.3). Different structures must not be mixed on the same axle (Construction and Use Regulations, regulation 26).
• Load and speed: the MOT checks the load rating and the speed rating only on vehicle classes 5 and 7, not on cars; on those classes, a load capacity insufficient for the axle weight is a dangerous defect (5.2.3).
• Fouling: a tyre fouling part of the vehicle at the time of the test is a major defect.
• Insurance: changing wheels is a modification that can affect your cover; check your policy and ask your insurer what to declare (Financial Ombudsman Service).

Your speedometer

The speedometer counts wheel revolutions. With a larger tyre, each revolution covers more road: at an indicated 50 mph, you are going faster. With a smaller tyre, it is the other way round. The calculation assumes a speedometer that is exact on the original tyres.

UN Regulation No. 39 requires a new vehicle's speedometer never to read below the true speed, and to read at most 10% of the true speed plus 4 km/h above it (regulation 35 of the Construction and Use Regulations refers to it). We found no text requiring that check to be repeated after a change of tyres; regulation 36 of the same Regulations requires the speedometer to be maintained in good working order when the vehicle is used on a road.

Frequently asked questions

How do you calculate a tyre's diameter?

Multiply the rim diameter by 25.4 to get millimetres, then add twice the sidewall height, which is the width multiplied by the aspect ratio. For a 205/55 R16: 406.4 + 2 × 112.75 = 631.9 mm.

What diameter difference is acceptable between two tyre sizes?

This calculator uses the ETRTO tolerance: ±3% of the original tyre's two sidewall heights, which is ±6.8 mm for a 205/55 R16. It is a geometric check: your vehicle handbook, the tyre placard and your insurer have the final say.

Can I fit a different tyre size in Great Britain?

The MOT treats tyres of different sizes or structures on the same axle as a major defect, and so does a tyre fouling part of the vehicle. Changing wheels or tyres is a modification that can affect your insurance: check your policy and ask your insurer what to declare.

Do front and rear tyres have to be the same size?

Not necessarily, but both tyres on the same axle do: different sizes on one axle are a major MOT defect. A staggered fitment, with different front and rear sizes, is possible when the vehicle maker lists it.

What rim width suits a 225/45 R17 tyre?

Our estimate gives a 7.5-inch measuring rim and a range of 7 to 8.5 inches, the figures Michelin publishes for this size. Your tyre maker's data sheet has the final say.

What happens to the speedometer with larger tyres?

It reads lower than your actual speed: with a 225/45 R17 instead of a 205/55 R16, 50 mph on the speedometer is 50.2 mph in reality. UN Regulation No. 39 requires a new vehicle's speedometer never to read below the true speed.

Further reading

• Wheel offset calculator
• Reading wheel dimensions (page in French)
• Rim, tyre and wheel: the difference (page in French)

Sources

• DVSA, MOT inspection manual for cars, section 5: axles, wheels, tyres and suspension, 5.2.3, GOV.UK.
• Road Vehicles (Construction and Use) Regulations 1986, regulation 26.
• Road Vehicles (Construction and Use) Regulations 1986, regulation 35.
• Road Vehicles (Construction and Use) Regulations 1986, regulation 36.
• Financial Ombudsman Service, car modifications and insurance.
• UN Regulation No. 30, passenger car tyres, paragraphs 2.20, 6.1.1.1 and 6.1.5, EUR-Lex.
• UN Regulation No. 39, speedometer equipment, paragraph 5.4, EUR-Lex.
• Michelin, Pilot Sport 4 S technical specifications, measuring rim and rim width range for each size.
• Michelin, CrossClimate 2 technical specifications.

Pages consulted on 10 October 2026. Rules change: your national authority has the final say.

Calculations and text: the Onyx Forged design office. Updated on 10 October 2026.

Your custom forged wheel project: design it.