TyreMaths

TyreMaths / Tyre size comparison

Tyre size comparison

Compare your current tyre size with a new one. See how much the overall diameter changes and what your speedometer will show.

How to read a tyre size

205/55 R16 91V
  • 205 – tyre width in millimetres
  • 55 – profile: the sidewall height as a percentage of the width (55% of 205 mm = 112.75 mm)
  • R – radial construction
  • 16 – wheel (rim) diameter in inches
  • 91V – load index and speed rating. Not used in this calculation, but a replacement must meet or beat the original.

How the maths works

Overall diameter = rim diameter (inches × 25.4) + 2 × sidewall height. For 205/55 R16 that is 406.4 + 225.5 = 631.9 mm. For 225/45 R17 it is 431.8 + 202.5 = 634.3 mm, which is 0.38% bigger.

A bigger tyre turns fewer times per mile, so the speedometer reads lower than your true speed. With these two sizes, an indicated 30 mph is about 30.1 mph.

What counts as safe?

The usual guide used by tyre fitters and manufacturers is to keep the overall diameter within 3% of the original size. This tool shows:

  • Green – within 3%, and no more than 1% bigger
  • Yellow – within 3% but more than 1% bigger, so the speedometer could start to under-read
  • Red – more than 3% different either way

UK rules (which follow UN ECE Regulation 39) say a speedometer must never show less than your true speed, and can over-read by up to 10% plus 4 km/h. Cars are usually set to read a little high, which gives some room for a slightly bigger tyre. Check with a GPS speed app after fitting.

Before you change size, also check

  • The size is approved for your car (handbook, door-pillar sticker or the manufacturer)
  • The load index and speed rating are at least as high as the original
  • The tyre clears the arches and suspension on full lock and with a full load
  • Your rims are within the tyre's approved rim-width range
  • Your insurer knows about the change – a different wheel or tyre size can count as a modification
  • All tyres on the same axle are the same size and type
This calculator uses nominal sizes. Real tyres vary by a few millimetres between brands and models, and with wear. Use the result as a guide, not as approval to fit.

Sources

Rules and figures last checked: November 2026. Sources are listed above so you can check them yourself.