31.2" tall, 12.0" wide, 6.6" sidewall
A 305/55R18 tire is 31.2 inches (793 mm) tall and 12.0 inches wide. The sidewall is 55% of the width, 6.6 inches, on a 18-inch wheel, and the tire turns about 646 times per mile.
In flotation terms that is about 31.2x12.0R18. People often call it a 31-inch tire.
The size most often compared with it is 285/60R18, which is 0.3" taller (+0.8%). At an indicated 60 mph you would be doing 60.5 mph on it.
| Current | New | Difference |
|---|
| Shows | Actual speed |
|---|
Same 18" wheel, within 20 mm of width and 10 points of aspect ratio.
| Size | Diameter | Width | Sidewall | vs 305/55R18 | |
|---|---|---|---|---|---|
| 285/60R18 | 31.5" | 11.2" | 6.7" | +0.8% | |
| 295/55R18 | 30.8" | 11.6" | 6.4" | −1.4% | |
| 295/60R18 | 31.9" | 11.6" | 7.0" | +2.3% | |
| 285/55R18 | 30.3" | 11.2" | 6.2" | −2.8% | |
| 305/60R18 | 32.4" | 12.0" | 7.2" | +3.8% | |
| 285/65R18 | 32.6" | 11.2" | 7.3" | +4.4% | |
| 33x12.50R18 | 33.0" | 12.5" | 7.5" | +5.7% | |
| 295/65R18 | 33.1" | 11.6" | 7.5" | +6.1% | |
| 285/50R18 | 29.2" | 11.2" | 5.6" | −6.4% | |
| 325/60R18 | 33.4" | 12.8" | 7.7" | +6.9% |
Sizes within 2% of 31.2" on 20" or 17" or 16" wheels.
| Size | Diameter | Width | Sidewall | vs 305/55R18 | |
|---|---|---|---|---|---|
| 285/50R20 | 31.2" | 11.2" | 5.6" | +0.0% | |
| 225/80R17 | 31.2" | 8.9" | 7.1" | −0.1% | |
| 315/45R20 | 31.2" | 12.4" | 5.6" | −0.2% | |
| 275/70R16 | 31.2" | 10.8" | 7.6" | −0.2% | |
| 275/65R17 | 31.1" | 10.8" | 7.0" | −0.4% | |
| 225/85R16 | 31.1" | 8.9" | 7.5" | −0.5% |
Actual speed when the speedometer is calibrated for 305/55R18.
| Speedometer shows | on 285/60R18 | on 295/55R18 | on 295/60R18 |
|---|---|---|---|
| 30 mph | 30.2 mph | 29.6 mph | 30.7 mph |
| 45 mph | 45.4 mph | 44.4 mph | 46.1 mph |
| 60 mph | 60.5 mph | 59.2 mph | 61.4 mph |
| 70 mph | 70.6 mph | 69.0 mph | 71.6 mph |
31.2 inches (793 mm). The sidewall is 6.6 inches.
12.0 inches (305 mm) section width, measured sidewall to sidewall.
About 31.2x12.0R18.
285/60R18 is +0.8% in diameter, within the common 3% guideline. It is 0.8" narrower.