29.9" tall, 12.4" wide, 5.0" sidewall
A 315/40R20 tire is 29.9 inches (760 mm) tall and 12.4 inches wide. The sidewall is 40% of the width, 5.0 inches, on a 20-inch wheel, and the tire turns about 674 times per mile.
In flotation terms that is about 29.9x12.4R20. People often call it a 30-inch tire.
The size most often compared with it is 305/40R20, which is 0.3" shorter (−1.1%). At an indicated 60 mph you would be doing 59.4 mph on it.
| Current | New | Difference |
|---|
| Shows | Actual speed |
|---|
Same 20" wheel, within 20 mm of width and 10 points of aspect ratio.
| Size | Diameter | Width | Sidewall | vs 315/40R20 | |
|---|---|---|---|---|---|
| 305/40R20 | 29.6" | 12.0" | 4.8" | −1.1% | |
| 295/45R20 | 30.5" | 11.6" | 5.2" | +1.8% | |
| 295/40R20 | 29.3" | 11.6" | 4.6" | −2.1% | |
| 305/45R20 | 30.8" | 12.0" | 5.4" | +3.0% | |
| 325/35R20 | 29.0" | 12.8" | 4.5" | −3.2% | |
| 315/45R20 | 31.2" | 12.4" | 5.6" | +4.1% | |
| 315/35R20 | 28.7" | 12.4" | 4.3" | −4.1% | |
| 295/50R20 | 31.6" | 11.6" | 5.8" | +5.7% | |
| 295/35R20 | 28.1" | 11.6" | 4.1" | −6.0% | |
| 305/50R20 | 32.0" | 12.0" | 6.0" | +7.0% |
Sizes within 2% of 29.9" on 18" or 22" wheels.
| Size | Diameter | Width | Sidewall | vs 315/40R20 | |
|---|---|---|---|---|---|
| 275/55R18 | 29.9" | 10.8" | 6.0" | −0.0% | |
| 285/35R22 | 29.9" | 11.2" | 3.9" | −0.2% | |
| 295/35R22 | 30.1" | 11.6" | 4.1" | +0.7% | |
| 285/55R18 | 30.3" | 11.2" | 6.2" | +1.4% | |
| 265/55R18 | 29.5" | 10.4" | 5.7" | −1.5% | |
| 315/30R22 | 29.4" | 12.4" | 3.7" | −1.6% |
Actual speed when the speedometer is calibrated for 315/40R20.
| Speedometer shows | on 305/40R20 | on 295/45R20 | on 295/40R20 |
|---|---|---|---|
| 30 mph | 29.7 mph | 30.5 mph | 29.4 mph |
| 45 mph | 44.5 mph | 45.8 mph | 44.1 mph |
| 60 mph | 59.4 mph | 61.1 mph | 58.7 mph |
| 70 mph | 69.3 mph | 71.2 mph | 68.5 mph |
29.9 inches (760 mm). The sidewall is 5.0 inches.
12.4 inches (315 mm) section width, measured sidewall to sidewall.
About 29.9x12.4R20.
305/40R20 is −1.1% in diameter, within the common 3% guideline. It is 0.4" narrower.