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Tyre size calculator and comparison

Compare diameter, ride height and theoretical speed for your current and proposed tyre sizes. These calculations explain a size change; they do not confirm that the new tyre is approved or fits your car.

Input values

Values are processed in your browser, without sending them to a calculation service or saving them. Reloading clears the form.

Scope and limits

This is geometry, not a fitment or roadworthiness approval. Load and speed ratings, rim width, body clearance, tyre pressure, wear and manufacturer approvals are not assessed. Have compatibility checked before fitting.

How to use the result

Enter your existing size as A and the proposed size as B. Positive diameter difference means a larger tyre; the height change is half the diameter difference. The speed estimate assumes the original setup was correctly calibrated.

Method and assumptions

Read 205/55 R16 as 205 mm width, 55% sidewall ratio and a 16-inch rim. Diameter = rim inches × 25.4 + 2 × width × ratio ÷ 100. Circumference = π × diameter.

Worked example

Input values

Width A (mm)
205
Sidewall ratio A (%)
55
Rim diameter A (inches)
16
Width B (mm)
225
Sidewall ratio B (%)
45
Rim diameter B (inches)
17
Reference speed on original tyres (km/h)
100

Result of this example

Diameter A
631.9 mm
Diameter B
634.3 mm
Circumference A
1,985.17 mm
Circumference B
1,992.71 mm
Diameter difference
0.38 %
Theoretical ride-height change
1.2 mm
Speed at the same wheel rotation rate
100.38 km/h

Put several tyre sizes on the same shortlist

Keep one reference and compare up to five candidates. The radius column shows the theoretical ride-height change for each size.

Your entries stay in this browser tab. Nothing is sent to Osmopark.

Worked example and its assumptions
Reference tyre, e.g. 205/55 R16
205/55 R16
Candidates: one size per line, maximum 5
195/65 R15
225/45 R17
225/40 R18
SizeDiameter (mm)Circumference (mm)Diameter change (%)Radius change (mm)
195/65 R15634.51,993.340.411.3
225/45 R17634.31,992.710.381.2
225/40 R18637.22,001.820.842.65

Nominal unloaded geometry only. Check the manufacturer’s approved combinations, rim width, axle loads, speed index and clearance before fitting.

Sources and scope checked on 31 October 2026: Continental

Read the whole sidewall, not just the size

These three fictional label examples separate dimensions from the service description. The numbers after the rim size are load indices, not kilograms. The final letter is a speed category, not permission to drive at that speed.

Example markingWhat to check on the vehicle documents
205/55 R16 91V205 mm width, 55% aspect ratio, radial construction, 16-inch rim. Compare 91 and V with the approved service description.
225/45 R17 94W XLXL belongs to the load specification. It does not mean any heavier vehicle can use this size, and does not set the correct inflation pressure.
195/65 R15 91H · M+S · 3PMSFCheck each symbol on the actual tyre. A winter symbol does not replace checks of approved size, load capacity or the rules for your route.

Sources and scope checked on 31 October 2026: Continental

Sources and review

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