Tyre Size Calculator

How to use this calculator

1. Read the marking off the sidewallIt is moulded into the tyre — three numbers separated by a slash and an R. 205/55R16 gives 205, 55 and 16.
2. Remember the middle number is a percentageThe aspect ratio is a percentage of the width, not a measurement. A 205/55 and a 225/55 have different sidewall heights despite both saying 55.
3. Enter a second size to compareThe comparison shows diameter difference, ride height change, speedometer error and whether the change sits inside the usual ±3% tolerance.
4. Check against the V5C and the door pillarThe manufacturer's approved sizes are listed on the vehicle. Fitting something outside that list can affect insurance and may be a notifiable modification.

Three unit systems in one code

A tyre marking is a historical accident that mixes metric and imperial in the same string, which is why it confuses people.

The first number is the section width in millimetres — the width of the tyre at its widest point, not the width of the tread. 205 means 205 mm.

The second number is the aspect ratio, expressed as a percentage of the section width. 55 means the sidewall height is 55% of 205 mm, which is 112.75 mm. This is the part most often misread as a measurement.

The letter indicates construction. R is radial, which is effectively universal on modern cars. You may occasionally see ZR on high-speed-rated tyres.

The last number is the rim diameter in inches. 16 means a 16-inch wheel, or 406.4 mm.

Overall diameter is the rim plus two sidewalls, since the sidewall appears at the top and the bottom: 406.4 + 2 × 112.75 = 631.9 mm.

The consequence of the aspect ratio being a percentage is that widening a tyre without changing the aspect ratio also makes it taller. Going from 205/55R16 to 225/55R16 adds 20 mm of width and 11 mm of sidewall, raising the overall diameter by 22 mm — a 3.5% change that pushes past the usual tolerance. This is why size changes are normally made in pairs: widen the tyre and reduce the aspect ratio, or go up a rim size and reduce the sidewall, so the overall diameter stays close to standard.

Worked example: 205/55R16 to 225/45R17

The original: sidewall is 205 × 0.55 = 112.75 mm. Diameter is 16 × 25.4 + 2 × 112.75 = 406.4 + 225.5 = 631.9 mm. Circumference is π × 631.9 = 1,985.2 mm, so the wheel turns 503.7 times per kilometre.

The replacement: sidewall is 225 × 0.45 = 101.25 mm. Diameter is 17 × 25.4 + 2 × 101.25 = 431.8 + 202.5 = 634.3 mm.

The difference is 2.4 mm, or 0.38% — comfortably inside the ±3% guideline. Ride height rises by half that, 1.2 mm, since the diameter change is shared between top and bottom.

This is a textbook plus-one upgrade: a bigger rim, a wider tyre, a much shorter sidewall, and an overall diameter almost unchanged. The car looks different, the handling changes, and the speedometer stays honest.

Speedometer effect: at an indicated 70 mph the true speed becomes 70 × 634.3 ÷ 631.9 = 70.27 mph. The speedo under-reads by a quarter of a mile per hour, which is negligible.

Now contrast a careless change. Fitting 225/55R16 instead — same rim, wider tyre, aspect ratio left alone — gives a sidewall of 123.75 mm and a diameter of 654.0 mm. That is 22.1 mm larger, 3.5%, outside the tolerance. At an indicated 70 mph the true speed would be 72.4 mph, so you would be doing 72 in a 70 while the dial read 70. Over a year the odometer would under-record by 3.4 miles in every hundred.

That is the practical reason to care about the arithmetic rather than the appearance.

Common sizes and their diameters

Notice how close the middle four are despite looking very different — all within about 6 mm of each other. That is not coincidence: they are the standard plus-sizing progression for the same vehicle platform, each pairing a larger rim with a shorter sidewall to hold the rolling diameter constant. The last row is 3.5% larger than the 205/55R16 and would introduce a real speedometer error.

Speedometers, the law and what else changes

A speedometer works by counting wheel revolutions and multiplying by an assumed circumference. Change the circumference and the reading is wrong, in a direction that surprises people: a larger tyre travels further per revolution, so the car is going faster than the dial says.

UK regulations require that a speedometer never under-reads. It may over-read by up to 10% plus 6.25 mph, which is why most cars show a speed slightly above true — a genuine 70 mph often indicates around 73. That built-in margin means a small increase in tyre diameter usually eats into the manufacturer's over-read rather than making the speedo dangerously optimistic. A large increase can consume it entirely.

The odometer is affected by the same percentage, which matters for service intervals, warranty mileage limits, lease agreements and resale.

Other systems assume the original diameter too. ABS and traction control compare wheel speeds; a mismatch between axles can confuse them. Cruise control and speed limiters work off the same signal. Modern driver assistance systems calibrated to wheel speed inherit the error.

On the legal and insurance side: tyres must be the correct size and specification for the vehicle, with a load index and speed rating at least matching the manufacturer's requirement. Fitting a lower speed rating or load index is genuinely unsafe and can invalidate insurance. Non-standard sizes may count as a modification your insurer needs to know about.

The minimum legal tread depth in the UK is 1.6 mm across the central three quarters of the tread and around the entire circumference, and the penalty is per tyre. Performance in the wet degrades well before that point, and many manufacturers recommend replacement at 3 mm.

All four tyres should ideally match. Mixing sizes across an axle is unlawful in most configurations, and mixing radial and cross-ply is prohibited. On a four-wheel-drive car, even small circumference differences between axles can damage the transmission.

Winter and all-season tyres use the same marking system, with an additional snowflake symbol for severe snow certification.

Assumptions and limitations

• Uses nominal marking geometry — real loaded rolling diameter is slightly smaller due to deflection
• Manufacturers vary within tolerance, so measured circumference can differ by a percentage or so
• The ±3% guideline is an industry convention, not a statutory limit
• Does not check load index or speed rating, both of which must meet the manufacturer's requirement
• Does not verify whether a size is approved for your vehicle — check the V5C and door pillar
• Speedometer figures ignore the manufacturer's built-in over-read, which absorbs small changes
• Not a substitute for a tyre fitter's advice on what is legal and safe for your car

Questions people ask about this in United Kingdom