27.2" tall, 12.0" wide, 3.6" sidewall
A 305/30R20 tire is 27.2 inches (691 mm) tall and 12.0 inches wide. The sidewall is 30% of the width, 3.6 inches, on a 20-inch wheel, and the tire turns about 741 times per mile.
In flotation terms that is about 27.2x12.0R20. People often call it a 27-inch tire.
The size most often compared with it is 275/35R20, which is 0.4" taller (+1.4%). At an indicated 60 mph you would be doing 60.8 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 305/30R20 | |
|---|---|---|---|---|---|
| 295/30R20 | 27.0" | 11.6" | 3.5" | −0.9% | |
| 285/35R20 | 27.9" | 11.2" | 3.9" | +2.4% | |
| 295/35R20 | 28.1" | 11.6" | 4.1" | +3.4% | |
| 315/35R20 | 28.7" | 12.4" | 4.3" | +5.4% | |
| 325/35R20 | 29.0" | 12.8" | 4.5" | +6.4% | |
| 295/40R20 | 29.3" | 11.6" | 4.6" | +7.7% | |
| 305/40R20 | 29.6" | 12.0" | 4.8" | +8.8% | |
| 315/40R20 | 29.9" | 12.4" | 5.0" | +10.0% |
Sizes within 2% of 27.2" on 18" or 19" wheels.
| Size | Diameter | Width | Sidewall | vs 305/30R20 | |
|---|---|---|---|---|---|
| 295/40R18 | 27.3" | 11.6" | 4.6" | +0.3% | |
| 255/45R18 | 27.0" | 10.0" | 4.5" | −0.6% | |
| 285/40R18 | 27.0" | 11.2" | 4.5" | −0.8% | |
| 285/35R19 | 26.9" | 11.2" | 3.9" | −1.3% | |
| 275/45R18 | 27.7" | 10.8" | 4.9" | +2.0% | |
| 275/40R18 | 26.7" | 10.8" | 4.3" | −2.0% |
Actual speed when the speedometer is calibrated for 305/30R20.
| Speedometer shows | on 275/35R20 | on 295/30R20 | on 285/35R20 |
|---|---|---|---|
| 30 mph | 30.4 mph | 29.7 mph | 30.7 mph |
| 45 mph | 45.6 mph | 44.6 mph | 46.1 mph |
| 60 mph | 60.8 mph | 59.5 mph | 61.4 mph |
| 70 mph | 71.0 mph | 69.4 mph | 71.7 mph |
27.2 inches (691 mm). The sidewall is 3.6 inches.
12.0 inches (305 mm) section width, measured sidewall to sidewall.
About 27.2x12.0R20.
275/35R20 is +1.4% in diameter, within the common 3% guideline. It is 1.2" narrower.