Compare two tire sizes: overall diameter, circumference, revolutions per km and speedometer error.
Overall diameter = rim diameter + 2 × sidewall height, where sidewall = width × aspect ratio. A 205/55R16 is 631.9 mm tall and a 225/45R17 is 634.3 mm — a 0.38% difference, so the speedometer error is under 0.4%.
The Tire Size Calculator decodes the numbers on a tyre sidewall and compares two sizes. It shows each tyre's sidewall height, overall diameter, circumference and revolutions per kilometre and mile, and the difference between them. It also shows how the speedometer and odometer would read if you fit the new size.
Use it when choosing plus-size alloy wheels, replacing tyres with a different size, or checking that a change keeps the overall diameter close to the original (usually within about 2–3%). Check your vehicle manual for approved sizes, load and speed ratings. See also the Speed Calculator.
The size code gives three numbers: width in millimetres, aspect ratio and rim diameter in inches.
| Tire A | Tire B | Diameter change | Actual speed when speedo reads 100 |
|---|---|---|---|
| 205/55R16 | 225/45R17 | +0.38% | 100.38 |
| 185/65R15 | 195/60R16 | +3.04% | 103.04 |
| 195/65R15 | 205/60R16 | +2.82% | 102.82 |
| 205/55R16 | 215/60R16 | +5.14% | 105.14 |
| Part | Meaning |
|---|---|
| 205 | Section width in millimetres |
| 55 | Aspect ratio: sidewall height as a % of width |
| R | Radial construction |
| 16 | Rim (wheel) diameter in inches |
A change of more than about 3% in overall diameter can affect the speedometer, odometer, gearing, ABS and traction-control calibration, and may cause wheel-arch rubbing. A new tyre should also match or exceed the load index and speed rating of the original.
In 205/55R16, 205 is the width in millimetres, 55 is the sidewall height as a percentage of width, R means radial and 16 is the rim diameter in inches.
Add the rim diameter in millimetres (inches × 25.4) to twice the sidewall height (width × aspect ratio ÷ 100).
Keep the new overall diameter within about 2–3% of the original so the speedometer and gearing stay accurate.
A larger overall diameter covers more road per revolution, so the speedometer reads lower than your true speed. A smaller one reads higher.
It matches the diameter change: if the new tyres are 2% larger, the odometer under-reads distance by about 2%.
Fitting a larger rim with a lower-profile tyre so the overall diameter stays about the same, for example 205/55R16 to 225/45R17.
Often within a small range. Check the rim's approved width range and the clearance, and consult the tyre maker.
P stands for passenger and LT for light truck; Z or ZR indicates a high-speed rating.
How many times the tyre turns to cover a kilometre. Fewer revolutions per km means a larger tyre.
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