How do I read a tyre size?

In 225/65R17, 225 is the width in mm, 65 is the sidewall height as a percentage of the width, R means radial, and 17 is the wheel diameter in inches.

How much diameter change is acceptable?

Many mechanics advise keeping it within about 3% of the original to limit speedometer, ABS and gearing effects.

Will bigger tyres make my speedometer read low?

Yes. A larger overall diameter covers more ground per turn, so the speedometer under-reads your true speed.

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Tire Size Calculator

Enter your current and new tyre sizes to see the difference in overall diameter and what your speedometer will read. A size like 225/65R17 means 225 mm wide, a sidewall 65% of the width, and a 17-inch wheel.

Current vs New Tire

Current Tire — width / aspect / rim

New Tire — width / aspect / rim

Old diameter

28.52"

New diameter

26.33"

Height change

-7.7%

New sidewall

4.16"

Speedometer error

When your speedometer reads 60 mph, you are actually going 55.4 mph

When your speedometer says 60 mph, your true speed with the new tyres is shown above.

Reads 30 mph

27.7 mph

Reads 60 mph

55.4 mph

Reads 100 mph

92.3 mph

The diameter difference is over 3% — expect a noticeable speedometer/odometer error and possible ABS or gearing effects.

Compare two tire sizes to see differences in diameter, sidewall height and width, and how much your speedometer and odometer readings shift.

Formula

Tire diameter = rim + 2 × sidewall, where sidewall (in) = width (mm) × aspect ÷ 100 ÷ 25.4. Speedometer error % = (new ÷ old − 1) × 100.

Reference Values

•235/45R18 diameter ≈ 26.3 in
•225/65R17 diameter ≈ 28.0 in
•Keep diameter within ±3% of stock

FAQ

Common questions about Tire Size Calculator

How we calculate this

We use the standard published formula for this calculation, and we show the working so you can check it.

Spotted a mistake? Tell us and we'll check it within a few days. See how we check our calculators.