Tyre Size Calculator guide
Tyre Size Calculator: Dimensions and Speedometer Difference
Use our tyre size calculator to decode tyre sizes like 205/55 R16 and compare two sizes to see the diameter and speedometer difference.
By the AllCalculator editorial teamUpdated 16 October 20261 min read
The numbers on a tyre's sidewall tell you its exact size. Our Tyre Size Calculator decodes them and shows what happens if you change to a different size.
How to use the calculator (Tyre Size Calculator)
- Enter your current tyre size, like 205/55 R16.
- Optionally, enter a new tyre size to compare.
You'll see the sidewall height, overall diameter, circumference and revolutions per mile, and the speedometer difference.
Reading a tyre size
In 205/55 R16:
- 205 is the tread width in millimetres.
- 55 is the profile: the sidewall height is 55% of the width.
- R means radial construction.
- 16 is the wheel rim diameter in inches.
The formulas
- Sidewall = width × profile ÷ 100
- Overall diameter = 2 × sidewall + rim × 25.4mm
- Circumference = π × diameter
A worked example
With the calculator's default settings, a 205/55 R16 tyre has a sidewall of 112.8mm and an overall diameter of 631.9mm. A 225/45 R17 is 634.3mm, just 0.38% bigger, so at an indicated 70 mph you'd be doing about 70.3 mph.
Frequently asked questions
How much can the diameter change?
Try to stay within about 3% of the original, so your speedometer, ABS and stability control work properly. Check your vehicle handbook.
What's the legal tread depth?
In the UK, at least 1.6mm across the central three-quarters of the tread, around the whole tyre.
Do I need to tell my insurer?
Yes, if you fit wheels or tyres that are different from the manufacturer's standard options.
Sources and further reading
Links open official and independent websites in a new tab.
This guide is general information to help you understand your options. It isn't financial, tax, legal or building advice and doesn't take your personal circumstances into account. Figures were checked when the guide was last updated and can change, so confirm anything important with GOV.UK, HMRC, a regulated adviser or a qualified tradesperson.