225/45R18 tire size
| Section width225 mm | 8.86in |
|---|---|
| Sidewall height101.3 mm — 45% of section width | 3.99in |
| Overall diameter | 25.97in |
| Circumference | 81.59in |
| Revolutions per mile | 777rev/mi |
| Wheel diameter | 18in |
| Vs. a 55-series on the same wheel | -6.4% |
Notes
- Speedometer error comes from diameter, not width. Fitting a tire 6.4% shorter than stock makes an indicated 60 mph read about 56.2 mph in reality.
- Section width is the widest point of the unloaded sidewall, not the tread width. Tread is typically 15–25 mm narrower, and both grow slightly on a wider wheel.
- Keep a replacement within ±3% of the original diameter. Beyond that, ABS and traction control read wheel speed wrong and speedometer error becomes noticeable.
Reading the size
The three numbers are not the same kind of measurement, which is why the size looks confusing. 225 is the section width in millimetres. 45 is a percentage, not a length — the sidewall is 45% of the section width, which here works out to 3.99″. 18 is the wheel diameter in inches.
Add two sidewalls to the wheel and you get the overall diameter: 25.97 inches.
Why revolutions per mile matters
At 777 revolutions per mile, this tire size is what your speedometer and odometer are calibrated against. Change the diameter and both read wrong by the same percentage — the speedometer does not measure road speed, it counts wheel rotations.
Fitting a different size
Stay within about 3% of the original overall diameter. Inside that band, ABS and traction control still see wheel speeds they expect. Outside it, the corrections they apply are based on a tire that is no longer on the car.
Nearby sizes
Read more
- Load Index and Speed Rating — The Two Codes at the End of the SizeEveryone reads the size and stops. The two characters after it are the ones with a safety consequence.
- Speedometer Error From Tire Size — One Ratio, and It Runs Both WaysA one-inch increase in diameter on a 30-inch tire is a 3.3% error. At an indicated 70 you are doing a bit over 72.
- Recalibrating a Speedometer After a Tire Change: Four Routes, and How to Verify It WorkedOn most modern vehicles the tire size is a stored number, not a mechanical ratio. Where that is true, correcting it takes minutes; where it is not, the options get physical.
- Plus Sizing Chart: Wider Tire, Shorter Sidewall, Same Rolling DiameterPlus sizing trades sidewall for wheel and keeps the overall diameter where it was. Done properly nothing on the vehicle notices — which is the entire point.