How to read a metric tyre or tire size
In 225/45 R17, the tread-section width is 225 mm and the sidewall is 45% of that width. The letter R means radial construction, while 17 identifies the wheel diameter in inches.
The numbers and letters that follow the size usually contain the load index and speed rating. They matter just as much as diameter when checking a replacement.
Tyre diameter and rolling-circumference formula
Rolling circumference is then estimated as π × overall diameter. The result is theoretical because the loaded rolling radius is not identical to nominal geometry.
Common tyre size comparison examples
| Current tyre | New tyre | Diameter change | Estimated actual speed |
|---|---|---|---|
| 205/55 R16 | 225/45 R17 | +0.38% | 70.3 mph |
| 225/40 R18 | 235/35 R19 | +1.55% | 71.1 mph |
| 195/65 R15 | 205/55 R16 | −0.41% | 69.7 mph |
These examples explain geometry only. Enter your own sizes in the calculator rather than relying on a generic plus-sizing chart.
Why tyre size changes speedometer error and ride height
A larger circumference travels farther on every revolution. If the vehicle continues to count wheel revolutions in the same way, actual road speed becomes higher than it was with the original tyre at the same dashboard reading.
The theoretical ride-height change equals half the diameter difference. Moving from a 16-inch to a 17-inch wheel does not necessarily raise the vehicle because a lower-profile sidewall can offset the larger wheel.
Tyre equivalence is not the same as approved fitment
Two sizes can have nearly identical diameters and still be unsuitable for the same car. Wheel width and offset, brake package, suspension clearance, axle load and electronic systems all affect fitment.
Sources and checks
AutoPrisme uses the figures you enter. The links below explain the rule, market context or consumer protections used to frame this calculator. Always verify the numbers in your own quote or contract.
- GOV.UK — MOT inspection manual: wheels and tyresOfficial UK inspection guidance covering tyre size, structure, load capacity, speed rating and incompatible fitment.
- NHTSA — Tire safetyUS road-safety guidance on tyre identification, loading, maintenance and replacement.
Frequently asked questions
What does 225/45 R17 mean?
225 is the nominal tyre width in millimetres, 45 means the sidewall height is 45% of that width, R identifies radial construction and 17 is the wheel diameter in inches.
How do I calculate the overall tyre diameter?
Multiply the width by the aspect ratio, divide by 100, double the sidewall result and add the wheel diameter converted from inches to millimetres.
How does tyre size affect speedometer accuracy?
A larger rolling circumference covers more distance per wheel revolution, so the theoretical actual speed rises for the same indicated speed. A smaller circumference has the opposite effect.
Is a tyre legal if the diameter difference is small?
Not automatically. A close geometric result does not confirm vehicle approval, road legality, wheel compatibility, load index, speed rating or physical clearance.
Can I compare US tire sizes with this calculator?
Yes for common metric sidewall markings such as 225/45 R17. The tool accepts speed in mph or km/h, but it does not currently convert flotation sizes such as 33×12.50 R15.
Does the calculation include tyre wear and deformation?
No. It uses nominal geometry. Pressure, tread wear, load, manufacturing tolerances and deformation can change the real rolling radius.