33x12.50R18 measures 33.0″ overall diameter and 12.5″ section width. Sidewall height is 7.5″, circumference is 103.7″, and the tire turns 611 times per mile. Those figures are geometry off the stamped size, not a brand or load range.
33x12.50R18 dimensions
| Measurement | Inches | Metric |
|---|---|---|
| Overall diameter | 33.0″ | 838 mm |
| Section width | 12.5″ | 318 mm |
| Sidewall height | 7.5″ | 191 mm |
| Circumference | 103.7″ | 2633 mm |
| Revolutions per mile | 611 | 380 / km |
This is a flotation size. The first number is the overall diameter (33.0″), the second is the section width (12.5″), and the last is the rim (18 inches). Sidewall height is half of (diameter minus rim): 7.5″.
Sizes near 33x12.50R18
A swap that stays inside about 3% diameter usually keeps the speedometer honest and clears the same well. 275/70R18 is a close neighbour at +0.5% diameter. The percent column opens both stamps in the comparison calculator.
| Size | Diameter | Width | vs 33x12.50R18 |
|---|---|---|---|
| 275/70R18 | 33.2″ | 10.8″ | +0.5% |
| 265/70R18 | 32.6″ | 10.4″ | -1.2% |
| 285/65R18 | 32.6″ | 11.2″ | -1.3% |
| 33x12.50R15 | 33.0″ | 12.5″ | 0.0% |
| 33x12.50R17 | 33.0″ | 12.5″ | 0.0% |
| 33x12.50R20 | 33.0″ | 12.5″ | 0.0% |
How to read the stamped size
33x12.50R18 is a flotation size, common on trucks and off-road tires. The first number is already the overall diameter in inches, so there is no aspect-ratio step. See how to read tire size for the metric form and 33 vs 35 tires for the usual flotation pair.
Diameter and width tell you whether a size is in the same family as what you have. They do not tell you whether 33x12.50R18 clears your fenders, matches your wheel width, or is load-rated for the vehicle. Check the door-jamb sticker and the wheel well before you buy.
Speedometer and revs
At 611 revolutions per mile, a trip computer calibrated for 33x12.50R18 is reading true only while this diameter is on the truck. A taller replacement under-reads speed and distance by the same percent the diameter grew. A shorter one over-reads. The comparison calculator puts a second size next to 33x12.50R18 and shows the error at 60 mph.