235/40R18 — the shortest 235 mm casing in the catalogue
A 235 mm tread on an 18-inch wheel with as little sidewall as the section width will support.
Metric notation · Performance and ultra-high performance
- Overall diameter
- 25.40 in
- 645 mm
- Section width
- 9.25 in
- 235 mm
- Sidewall height
- 3.70 in
- 94 mm
- Aspect ratio
- 40%
- sidewall as a share of width
- Wheel diameter
- 18 in
- the hole in the middle
- Circumference
- 79.8 in
- 2027 mm
- Revolutions per mile
- 794.0
- geometric, not measured
- Revolutions per km
- 493.4
- 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+7.60 in
- 35x12.50R1835.00 in tall+9.60 in
- 37x12.50R1837.00 in tall+11.60 in
Compare any two sizes, including ones this site has no page for, in the comparison tool.
Platforms recorded with this size
Nothing else carrying a 235 mm section stands shorter than this. The size belongs to sport compacts and small performance saloons — Golf R and GTI in their larger wheel options, Civic Type R, 1-series and A3 — where the wheel is as large as the arch allows and the tire is whatever is left.
Five points at a time, on this wheel
| Size | Aspect | Diameter | Sidewall |
|---|---|---|---|
| 235/40R18 | 40 | 25.40 in | 3.70 in |
| 235/45R18 | 45 | 26.33 in | 4.16 in |
| 235/50R18 | 50 | 27.25 in | 4.63 in |
Each five-point step adds 0.93 in of diameter and 0.46 in of sidewall. The steps are larger here than they are on a narrower casing, because the aspect ratio is a percentage of 235 mm rather than of 205 or 215 — the same two digits do more work as the tire gets wider.
Rim width, and why it matters more on a 40-series
3.70 in of sidewall is very little material to absorb a change in the angle at which the casing meets the rim, and this size sits at the point where that starts to be the dominant variable in how a tire looks on a car. The published 9.25 in describes the casing on one specified wheel; on anything else it is an approximation, and the shallower the sidewall the worse the approximation gets.
Two people measuring their own 235/40R18s on different wheel widths can get genuinely different numbers without either of them being wrong. The rim width to tire width reference has the relationship.
Its nearest match is two wheel sizes down
215/55R16 measures 25.31 in — 0.36% from this size, on a wheel two inches smaller, with 4.66 in of sidewall and 0.79 in less section width. Same rolling diameter, opposite design philosophy. 235/45R17 lands even closer at 25.33 in, 0.30% away, with the same 9.25 in of section.
What follows the size
Speed symbols in this size run high and some tires carry ZR in the size field, which is a marking rather than a dimension. The speed rating table has the scheme, and the comparison tool handles the geometry.
Related
- 225/45R17 vs 235/40R18
- 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.