235/40 R18 tyre size

Revolutions per kilometre

493.35

Revolutions per mile

793.97

Calculated from the nominal size, without load or tread wear. Revolutions use nominal circumference, not a measured rolling circumference. Tyre model, inflation, load and rim width can change the real dimensions.

235 is the nominal section width in millimetres, not tread width. 40 is the sidewall height as a percentage of that width. R means radial construction, and 18 is the rim diameter in inches. For the load index, speed rating and date code that follow the size, read how to read a tyre size.

7.5 to 8.5 inches

Our passenger-car guidance estimates a 8-inch measuring rim and a half-inch band either side. This is a starting estimate, not an approved rim-width range. The exact tyre manufacturer's published range takes priority. Our guide to choosing wheel width for a tyre explains how to find the approved range.

Computed candidates within ±3% of the 645.2 mm diameter, on 17-inch and 19-inch rims. Width stays within 20 mm of this tyre, using 165 to 355 mm widths in 10 mm steps and 25 to 80% profiles in 5% steps. These combinations are not a stock list or fitment approval; check availability and the exact tyre specification.

How a size change moves the speedometer and gearing is covered in plus-sizing wheels. A smaller rim must still clear the brakes. Check wheel width, offset, load and speed ratings, full steering and suspension travel, and vehicle restrictions. AWD and staggered cars may need a much tighter rolling-circumference match than this screening limit.

Use the trim, market and axle as well as the car name. Each guide links to its supporting sources and explains clearance limits.

Sizes people often weigh against 235/40 R18, with the full dimension, speedometer and gearing comparison on each page. The percentage is the change in overall diameter from 235/40 R18.

For how measured dimensions and approved rim ranges differ from nominal calculations, see Tire Rack’s tyre specification guide.