Tire Size Calculator: Equivalence and Speedometer Error

Tyre size equivalence: diameter, difference and speedometer error

A tyre’s diameter comes from its size marking (width / aspect ratio R rim): diameter = rim × 25.4 + 2 × width × aspect ratio / 100. Going from 205/55 R16 to 225/45 R17 adds 2.4 mm to the diameter (+0.38%): at 130 km/h on the speedometer you are doing 130.5 km/h. The tool accepts the new size if the difference stays between −2% and +1.5%. It also reads inch sizes (31x10.50R15 = 787.4 mm, close to 265/75 R15) and lists alternative sizes whose diameter stays within ±3%, the ITV tolerance in Spain.

Formulas

• Sidewall height = width × aspect ratio / 100 (205 × 0.55 = 112.8 mm).
• Diameter = rim in inches × 25.4 + 2 × sidewall.
• Circumference = π × diameter; revolutions per km = 1,000,000 / circumference.
• Actual speed = speedometer speed × new circumference / original circumference, assuming the speedometer is exact with the original tyres.
• Ground clearance change = diameter difference / 2.

Example

Difference: +2.4 mm, or +0.38%, ground clearance +1.2 mm. Speedometer readings of 50, 90, 110 and 130 km/h match 50.2, 90.3, 110.4 and 130.5 km/h actual. With 225/50 R17 the difference rises to +3.94%: 135.1 km/h actual for 130 shown, and the tool flags the size as out of tolerance.

Converting inch tyre sizes (31x10.50R15)

4x4 and pickup tyres are often sized in inches, in the "flotation" format: overall diameter x section width R rim. For 31x10.50R15 the diameter is 31 × 25.4 = 787.4 mm, the width 10.50 × 25.4 = 266.7 mm and the sidewall (31 − 15) / 2 = 8 inches, or 203.2 mm (aspect ratio 76.2). The nearest metric size on the same rim is 265/75 R15: 778.5 mm, or −1.13%. The tool takes the nearest metric width (ending in 5), then the aspect ratio, in steps of 5, that gives the closest diameter. The other way round, 205/55 R16 reads 24.9x8.07R16 (631.9 mm and 205 mm divided by 25.4).

Type the size in the "Tire size" field of either tyre: both formats are recognised, and the calculation keeps the exact inch dimensions. Going from 31x10.50R15 to 33x12.50R15 adds 6.45% to the diameter: 138.4 km/h actual for 130 shown.

Alternative tyre sizes within ±3% diameter

The tool tries widths from 155 to 335 mm (steps of 10), aspect ratios from 25 to 85 (steps of 5) and rims from 13 to 22 inches. It keeps the combinations whose diameter stays within ±3% of the original, sorts them by difference and shows 15, with a rim filter; by default the width stays within ±20 mm of the original. For 205/55 R16 (631.9 mm) the window runs from 612.9 to 650.9 mm and 79 sizes fit:

The list only checks the diameter. A combination from the grid is not necessarily made, and the load index, speed rating and rim width still need checking. The "−2/+1.5% rule" column shows whether the size also passes the stricter range used by the tool's verdict.

What the rules say in Spain, Germany and France

In Spain, the Manual de Procedimiento de Inspección de las Estaciones ITV (version 7.9, in force since 1 January 2026, section I, point 8.3) accepts tyres "equivalent" to those on the ITV card if they meet four conditions: load index at least the minimum on the ITV card or type approval, speed category at least equal, the same outer diameter within a ±3% tolerance, and a rim profile suited to the tyre. Version 7.6.0 from 2022 already said the same.

In Germany, the StVZO sets no percentage. Under § 19(2), the vehicle's operating permit (Betriebserlaubnis) lapses after a modification that can be expected to endanger other road users, unless the part has its own approval (§ 19(3)). The reference sizes are those in the registration certificate (Zulassungsbescheinigung Teil I) and the certificate of conformity (CoC); another size may need an inspection by a body such as TÜV or DEKRA and an entry in the papers.

In France, we found no official percentage. The roadworthiness test applies Directive 2014/45/EU, whose Annex I, point 5.2.3(a), lists tyres whose size, load capacity or speed rating do not meet the requirements. The reference remains the list of sizes specified for the vehicle (certificate of conformity, door-jamb placard); a size missing from those documents may require approval. In France and Germany, ±3% is therefore only a common rule of thumb.

Assumptions and limits

The tool's verdict (green or red) follows the −2% to +1.5% range: a rule of thumb from the tyre trade, stricter than the ±3% of the Spanish ITV, not a legal rule. The list of alternative sizes goes up to ±3%. Load and speed indices and rim compatibility are not checked, and the tool computes a theoretical diameter, while the real one varies with brand, wear and pressure. UN Regulation No. 39 requires that a speedometer never shows less than the actual speed, so a factory speedometer usually reads slightly high: the figures here show the error added by the tyre change, not the speed a GPS would show.

Sources and dates: Manual de Procedimiento de Inspección de las Estaciones ITV, version 7.9 (Spanish Ministry of Industry, in force 01/01/2026, section I, point 8.3); § 19 StVZO (Germany); Directive 2014/45/EU, Annex I, point 5.2.3; UN Regulation No. 39 (speedometer equipment), cited in text; tyre size designation per ETRTO / ISO standards. (checked 22/10/2026)

Frequently Asked Questions

How do I calculate the diameter of a 205/55 R16 tyre?

Sidewall: 205 × 0.55 = 112.75 mm. Diameter: 16 × 25.4 + 2 × 112.75 = 631.9 mm, giving a circumference of 1,985 mm and about 504 wheel turns per kilometre.

What diameter difference does the tool accept?

The tool's verdict accepts the new size between −2% and +1.5% of the original diameter, a trade rule of thumb stricter than the ±3% of the Spanish ITV. The list of alternative sizes goes up to ±3%. France and Germany set no percentage: the reference remains the list of sizes specified for the vehicle.

With bigger tyres, am I driving faster than shown?

Yes: actual speed rises in the same ratio as the circumference. With a +3.94% diameter (205/55 R16 to 225/50 R17), 130 km/h on the speedometer is 135.1 km/h actual.

Does my speedometer show the exact speed with the original tyres?

Rarely: UN Regulation No. 39 forbids showing less than the actual speed, so manufacturers set speedometers slightly high. The tool only calculates the error added by the size change.

How do I convert a 31x10.50R15 tyre to a metric size?

31 inches × 25.4 = 787.4 mm in diameter and 10.50 × 25.4 = 266.7 mm in width, on a 15-inch rim. The nearest metric size is 265/75 R15: 778.5 mm, or 1.13% smaller.

What diameter tolerance does the ITV accept in Spain?

±3% compared with the tyre on the ITV card, under point 8.3 of the Manual de Procedimiento de Inspección de las Estaciones ITV (version 7.9, in force since 1 January 2026). The load index and speed category must be at least equal, and the rim must suit the tyre.

Does a different tyre size need registering in Germany?

Often: if the size is in neither the Zulassungsbescheinigung Teil I nor the CoC, the change can void the operating permit (§ 19 StVZO), and an inspection by a body such as TÜV or DEKRA followed by an entry in the papers is then needed. The StVZO sets no diameter percentage.

What alternative sizes fit a 205/55 R16 tyre?

Within ±3% diameter (612.9 to 650.9 mm) and ±20 mm width, the tool finds 79, including 195/65 R15 (+0.41%), 225/45 R17 (+0.38%) and 215/40 R18 (−0.43%). Then check load and speed indices and the sizes specified for your vehicle.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.