Tyre Size Calculator

Original width, mm

Original profile, %

Original rim, inches

New width, mm

New profile, %

New rim, inches

Result

VerdictAn acceptable swap: within the 3% tolerance
Difference in diameter, %0.38%
Original diameter, mm631.9
New diameter, mm634.3
Change in ride height, mm1.2
Real speed at an indicated 100100.4
New circumference, mm1,993

Three numbers on the sidewall and what they mean

Compares two tyre sizes by overall diameter, shows the change in ride height and the speedometer error, and says whether the swap stays inside the usual three percent tolerance.

A marking of 205/55 R16 reads as 205 millimetres wide, a sidewall 55 percent of that height, and a 16-inch rim. The overall diameter falls straight out of those three numbers: the rim in millimetres plus two sidewalls.

The rule for swapping is to stay inside three percent on overall diameter. A speedometer counts wheel revolutions rather than speed, so a taller tyre makes the dial read low, and the ABS, stability control and odometer all inherit the same error.

What this calculator shows

The overall diameter of both tyres and the gap between them as a percentage
Your real speed when the speedometer reads 100
The change in ride height — half the difference in diameter

What to keep in mind

The diameter comes from the marking geometry; a specific model can measure a couple of millimetres either way.
Three percent is the accepted limit: past it the speedometer error becomes noticeable and the electronics are fed the wrong speed.

FAQs

Can I fit a different tyre size?

Yes, if the overall diameter is within three percent and the width suits your rim. Tyre width should run about 1.2 to 1.4 times the rim width in millimetres.

How does tyre size affect the speedometer?

The instrument counts wheel revolutions and multiplies by the factory rolling circumference. A larger tyre covers more ground per turn, so your real speed is higher than shown and the odometer under-counts.

How much does ride height change?

By half the difference in diameter: 10 mm more overall diameter is 5 mm more ground clearance. The other half goes upward into the arch.

What does a low profile buy you?

Sharper steering and less roll, at the cost of ride quality and rims that bend on potholes. Below a 45 profile, bad roads punish wheels quickly.

Worked example

Going up an inch of rim

Input: 205/55 R16 → 225/45 R17

Output: 631.9 mm against 634.3 mm, 0.38% apart

Note: The classic pairing: the rim gains an inch, the sidewall gives one back, and the overall diameter barely moves. At an indicated 100 the error is under half a kilometre per hour.

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Tyre Size Calculator

Result

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