225/40R18 — the shallowest sidewall on an 18-inch wheel
An 18-inch wheel under a compact car, which leaves very little room for tire.
Metric notation · Performance and ultra-high performance
- Overall diameter
- 25.09 in
- 637 mm
- Section width
- 8.86 in
- 225 mm
- Sidewall height
- 3.54 in
- 90 mm
- Aspect ratio
- 40%
- sidewall as a share of width
- Wheel diameter
- 18 in
- the hole in the middle
- Circumference
- 78.8 in
- 2002 mm
- Revolutions per mile
- 803.9
- geometric, not measured
- Revolutions per km
- 499.5
- geometric, not measured
- Typical load index
- 88–104
- 1235–1984 lb per tire
Low-profile sizes hold less air for their width, so the same vehicle weight needs a higher reference inflation. Extra Load construction is common in this class, and that is what lifts the index band — not any change in the casing size.
The load-index range shown is the band this CLASS of size is typically produced in — not a list of what is in stock for this exact size today. Load index and speed symbol are printed on the sidewall of the specific tire you are holding, and the required minimum is on the placard of the specific vehicle you are fitting. Read both.
Nearest real sizes in imperial notation, same wheel
- 33x12.50R1833.00 in tall+7.91 in
- 35x12.50R1835.00 in tall+9.91 in
- 37x12.50R1837.00 in tall+11.91 in
Compare any two sizes, including ones this site has no page for, in the comparison tool.
Platforms recorded with this size
3.54 in of sidewall. No other 18-inch fitment in this catalogue runs less. Overall diameter is 25.09 in, section width 8.86 in, circumference 78.8 in, and the geometric revolutions-per-mile figure is 803.9.
The size belongs to compact and sport-compact cars fitted with the largest wheel their arches allow: Golf and A3 in their higher trims, Civic Si and Type R variants, 1-series, and a broad aftermarket population.
The 40-series ladder on an 18
| Size | Diameter | Section | Step |
|---|---|---|---|
| 225/40R18 | 25.09 in | 8.86 in | — |
| 235/40R18 | 25.40 in | 9.25 in | +0.31 in |
| 245/40R18 | 25.72 in | 9.65 in | +0.31 in |
| 255/40R18 | 26.03 in | 10.04 in | +0.31 in |
Ten millimetres of width buys 0.31 in of height at a 40-series aspect ratio — the smallest constant step of any family in this catalogue. Four sizes span 0.94 in of diameter and 1.18 in of section width, which is why 40-series ranges can offer several widths against the same speedometer calibration.
Small tire, large wheel
Of this size's 25.09 in of standing height, 18 in of it is wheel. Set that beside 195/65R15, which stands 24.98 in — near enough the same tire, arrived at with only 15 in of wheel and a sidewall of 4.99 in rather than 3.54 in.
Same rolling diameter, completely different proportion. That is the entire trajectory of passenger tire fitment over thirty years expressed in one comparison, and the plus sizing reference covers the reasoning behind it.
The consequences that are not geometric
A 3.54 in sidewall is the only compliant structure between an 18-inch rim and a pothole edge. It is also all the casing has to work with when the wheel behind it is wider or narrower than the one the 8.86 in figure was measured on, which is why two cars wearing this code can look less alike than the code implies.
Tires in this size commonly carry high speed symbols, listed in the speed rating table. For any pairing at all, use the comparison tool.
Related
- 205/55R16 vs 225/40R18
- 33x12.50R1833.00 in tall
- 35x12.50R1835.00 in tall
- 37x12.50R1837.00 in tall
Class: Performance and ultra-high performance. Every figure above is computed from the size code, not stored — see how these numbers are calculated.