Tyre Size Comparison Calculator

Compare an original and proposed tyre size side by side. Enter width, aspect ratio and wheel diameter for each — for example 225/45 R17 — to see sidewall, diameter, circumference, ride-height change and the effect on your speedometer reading.

Original tyre size

Width

Aspect

Wheel (in)

e.g. 225 / 45 R17 — enter width, aspect ratio and wheel diameter.

Proposed tyre size

Width

Aspect

Wheel (in)

e.g. 225 / 45 R17 — enter width, aspect ratio and wheel diameter.

Indicated speed (mph)

Comparison

Enter both tyre sizes to see the comparison.

How this calculator works

A tyre size such as 225/45 R17 describes three things: the tread width in millimetres (225), the sidewall height as a percentage of that width (45%), and the wheel diameter in inches (17). From those three figures the calculator works out the sidewall height, overall diameter and rolling circumference of each tyre, then compares them.

Sidewall height (mm) = width × aspect ratio ÷ 100
Diameter (mm) = wheel diameter (in) × 25.4 + 2 × sidewall height
Circumference (mm) = π × diameter
Diameter change % = (new diameter ÷ original diameter − 1) × 100
Ride-height change (mm) = (new diameter − original diameter) ÷ 2
Estimated road speed = indicated speed × new diameter ÷ original diameter

Worked example: 225/45 R17 gives a sidewall of 225 × 45 ÷ 100 = 101.25 mm and a diameter of 17 × 25.4 + 2 × 101.25 = 634.3 mm. Identical sizes give zero diameter and ride-height change.

What this does not tell you

These are nominal dimensions calculated from the size printed on the sidewall. Real tyres vary between brands and even between moulds of the same model, and the loaded (rolling) radius is smaller than the unloaded radius because the contact patch flattens under the car’s weight.

A similar overall diameter does not mean a tyre will fit your car. This tool cannot confirm wheel-arch clearance, suitable rim width for the tyre, load index, speed rating, or vehicle compatibility. Do not treat a size as “safe” simply because its diameter is close to the original — always check fitment with the tyre and wheel supplier before buying.

Frequently asked questions

Why does my speedometer change when I fit bigger tyres?

Most speedometers are calibrated to the original tyre’s rolling circumference. A larger tyre travels further per revolution, so at the same engine and wheel speed the car covers more ground — your true speed becomes higher than the speedometer shows. This calculator estimates that effect.

How much diameter change is acceptable?

There is no single safe figure. A common rule of thumb is to stay within about ±3% of the original diameter, but clearance depends on suspension travel, wheel width and offset, and the specific car. Treat any percentage as a starting point, not approval.

Why is the ride-height change half the diameter change?

The tyre sits below the axle, so only the lower half of the diameter change lifts the car. A 10 mm larger diameter raises the car by about 5 mm.

Are the results rounded?

Yes. Calculations use full precision internally and only the displayed values are rounded.

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