Reading a tyre size code
A code such as 205/55 R16 means a section width of 205 millimetres, an aspect ratio of 55 per cent and a rim diameter of 16 inches. The aspect ratio is the sidewall height as a percentage of the width, so the sidewall here is 112.75 millimetres.
Overall diameter = rim diameter in millimetres + two sidewalls. For 205/55 R16 that is 406.4 + 225.5 = 631.9 millimetres. The rolling circumference is the diameter multiplied by pi, which gives 1985 millimetres.
Worked example
Changing from 205/55 R16 to 215/55 R16 raises the diameter from 631.9 to 642.9 millimetres, an increase of 11.0 millimetres. The new circumference is 2020 millimetres and the speedometer reads 1.7 per cent low, so 70 mph on the dial is about 71.2 mph on the road. The new tyre turns about 797 times per mile.
These figures come from the size codes alone. A tyre of the same nominal size from different makers can vary slightly in real diameter.
What a size change affects
A larger diameter reduces the number of revolutions per mile, so the car travels slightly further per turn of the wheel. That changes the speedometer and odometer reading, the effective gearing and, at the margins, acceleration and fuel use.
The change also affects physical fitment: clearance to suspension, brakes and arches at full lock and full bump, rim width suitability, and whether the tyre is approved for the vehicle. Insurance terms can require the manufacturer's specified size, so check with your insurer before fitting a different one.
Choosing a size that keeps the error small
A common rule is to keep the new diameter within about 2.5 per cent of the original. A 1.7 per cent change keeps the speedometer close but is not zero, and the dial may already over-read slightly by design.
This calculator reports the arithmetic consequence of the codes you enter. It does not recommend a size, and it does not verify that the tyre is legal, correctly load rated or suitable for the rim.
Limits of this calculator
It uses nominal dimensions, so it does not model tread depth, wear, inflation or manufacturing tolerance. It does not check rim width, offset, brake clearance, load index, speed rating or vehicle approval.
For the speed error on its own, use the speedometer calculator. To check a reading against the manufacturer's pressure range, use the tyre pressure calculator.
To convert a dial reading with known diameters, use the UK speedometer calculator.
For US spelling and tire terminology, use the US tire size calculator.
Inputs for the tyre size calculator
Fitted tyre width
The first number on the sidewall in millimetres, such as 205 in 205/55 R16.
Fitted aspect ratio
The second number, the sidewall height as a percentage of the section width.
Fitted rim diameter
The figure after R in inches. Half-inch sizes such as 16.5 are accepted.
Alternative tyre width
Section width of the size you are considering, in millimetres.
Alternative aspect ratio
Aspect ratio of the alternative size, as printed on its sidewall.
Alternative rim diameter
Rim diameter of the alternative size in inches.
CALCULATOR QUESTIONS
Tyre size calculator UK questions
Does a bigger wheel always mean a bigger overall diameter?
No. A larger rim is usually paired with a lower aspect ratio, which keeps the overall diameter close to the original.
How much speedometer error is acceptable?
There is no single legal limit for a modified car, but keeping the diameter within about 2.5 per cent keeps the error small. Check your insurer's terms.
Can I use any tyre that fits the rim?
No. Load index, speed rating, rim width and vehicle approval all matter, and this calculator checks none of them.
Why does the circumference matter?
It sets how far the car travels per wheel revolution, which is what the speedometer and odometer are calibrated against.