25.4" tall, 12.4" wide, 3.7" sidewall
A 315/30R18 tire is 25.4 inches (646 mm) tall and 12.4 inches wide. The sidewall is 30% of the width, 3.7 inches, on a 18-inch wheel, and the tire turns about 793 times per mile.
In flotation terms that is about 25.4x12.4R18. People often call it a 25-inch tire.
The size most often compared with it is 305/30R18, which is 0.2" shorter (−0.9%). At an indicated 60 mph you would be doing 59.4 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 315/30R18 | |
|---|---|---|---|---|---|
| 305/30R18 | 25.2" | 12.0" | 3.6" | −0.9% | |
| 295/30R18 | 25.0" | 11.6" | 3.5" | −1.9% | |
| 295/35R18 | 26.1" | 11.6" | 4.1" | +2.7% | |
| 305/35R18 | 26.4" | 12.0" | 4.2" | +3.8% | |
| 295/40R18 | 27.3" | 11.6" | 4.6" | +7.3% |
Sizes within 2% of 25.4" on 20" or 16" or 17" wheels.
| Size | Diameter | Width | Sidewall | vs 315/30R18 | |
|---|---|---|---|---|---|
| 225/30R20 | 25.3" | 8.9" | 2.7" | −0.5% | |
| 245/50R16 | 25.6" | 9.6" | 4.8" | +0.8% | |
| 275/40R17 | 25.7" | 10.8" | 4.3" | +0.9% | |
| 315/35R17 | 25.7" | 12.4" | 4.3" | +0.9% | |
| 255/40R17 | 25.0" | 10.0" | 4.0" | −1.6% | |
| 185/55R17 | 25.0" | 7.3" | 4.0" | −1.7% |
Actual speed when the speedometer is calibrated for 315/30R18.
| Speedometer shows | on 305/30R18 | on 295/30R18 | on 295/35R18 |
|---|---|---|---|
| 30 mph | 29.7 mph | 29.4 mph | 30.8 mph |
| 45 mph | 44.6 mph | 44.2 mph | 46.2 mph |
| 60 mph | 59.4 mph | 58.9 mph | 61.6 mph |
| 70 mph | 69.4 mph | 68.7 mph | 71.9 mph |
25.4 inches (646 mm). The sidewall is 3.7 inches.
12.4 inches (315 mm) section width, measured sidewall to sidewall.
About 25.4x12.4R18.
305/30R18 is −0.9% in diameter, within the common 3% guideline. It is 0.4" narrower.