235/40R18 tires

What 235/40R18 means

The first number — 235 — is the tire's section width in millimeters (about 9.3 inches from sidewall to sidewall). The second number — 40 — is the aspect ratio: the sidewall height as a percentage of the section width. The R indicates radial construction (universal on passenger tires today), and 18 is the rim diameter in inches. Together these give an overall tire diameter of approximately 645.2 mm (25.4 inches), which is the dimension that matters for speedometer accuracy and clearance.

100 vehicle/year combinations in our catalog list this size as an OEM or approved fitment, and 13 tire models in our catalog are sold in this size. When replacing tires within a single size, the brand and compound choice are what change the driving experience — every tire in this size is engineered to the same outside diameter, so speedometer error and wheel clearance won't change. Where the differences show up is in tread compound (longer-wearing vs stickier), construction (touring sidewall vs performance-stiff), and season class.

If you are considering deviating from this size — a plus-size step up or a winter step down — keep the overall outside diameter within ±3% of the original. Major changes to outside diameter affect speedometer calibration, ABS reference, and AWD differentials on systems that rely on consistent tire revolutions per mile. Always confirm a non-OEM size with the manufacturer or a qualified tire shop before purchasing.