285/40R18 tire size
| Section width285 mm | 11.22in |
|---|---|
| Sidewall height114 mm — 40% of section width | 4.49in |
| Overall diameter | 26.98in |
| Circumference | 84.75in |
| Revolutions per mile | 748rev/mi |
| Wheel diameter | 18in |
| Vs. a 55-series on the same wheel | -11.1% |
- Section width
- 285 mm
- Aspect ratio
- 40 %
- Wheel diameter
- 18 in
Reading the size
The three numbers are not the same kind of measurement, which is why the size looks confusing. 285 is the section width in millimetres. 40 is a percentage, not a length — the sidewall is 40% of the section width, which here works out to 4.49″. 18 is the wheel diameter in inches.
Add two sidewalls to the wheel and you get the overall diameter: 26.98 inches.
Why revolutions per mile matters
At 748 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.
Notes
- Speedometer error comes from diameter, not width. Fitting a tire 11.1% shorter than stock makes an indicated 60 mph read about 53.3 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.