25.2" tall, 12.0" wide, 3.6" sidewall
A 305/30R18 tire is 25.2 inches (640 mm) tall and 12.0 inches wide. The sidewall is 30% of the width, 3.6 inches, on a 18-inch wheel, and the tire turns about 800 times per mile.
In flotation terms that is about 25.2x12.0R18. People often call it a 25-inch tire.
The size most often compared with it is 295/30R18, 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 305/30R18 | |
|---|---|---|---|---|---|
| 295/30R18 | 25.0" | 11.6" | 3.5" | −0.9% | |
| 315/30R18 | 25.4" | 12.4" | 3.7" | +0.9% | |
| 285/30R18 | 24.7" | 11.2" | 3.4" | −1.9% | |
| 285/35R18 | 25.9" | 11.2" | 3.9" | +2.6% | |
| 295/35R18 | 26.1" | 11.6" | 4.1" | +3.7% | |
| 305/35R18 | 26.4" | 12.0" | 4.2" | +4.8% | |
| 285/40R18 | 27.0" | 11.2" | 4.5" | +7.0% | |
| 295/40R18 | 27.3" | 11.6" | 4.6" | +8.3% |
Sizes within 2% of 25.2" on 20" or 17" or 16" wheels.
| Size | Diameter | Width | Sidewall | vs 305/30R18 | |
|---|---|---|---|---|---|
| 225/30R20 | 25.3" | 8.9" | 2.7" | +0.4% | |
| 255/40R17 | 25.0" | 10.0" | 4.0" | −0.7% | |
| 185/55R17 | 25.0" | 7.3" | 4.0" | −0.8% | |
| 225/50R16 | 24.9" | 8.9" | 4.4" | −1.4% | |
| 245/50R16 | 25.6" | 9.6" | 4.8" | +1.7% | |
| 275/40R17 | 25.7" | 10.8" | 4.3" | +1.8% |
Actual speed when the speedometer is calibrated for 305/30R18.
| Speedometer shows | on 295/30R18 | on 315/30R18 | on 285/30R18 |
|---|---|---|---|
| 30 mph | 29.7 mph | 30.3 mph | 29.4 mph |
| 45 mph | 44.6 mph | 45.4 mph | 44.2 mph |
| 60 mph | 59.4 mph | 60.6 mph | 58.9 mph |
| 70 mph | 69.3 mph | 70.7 mph | 68.7 mph |
25.2 inches (640 mm). The sidewall is 3.6 inches.
12.0 inches (305 mm) section width, measured sidewall to sidewall.
About 25.2x12.0R18.
295/30R18 is −0.9% in diameter, within the common 3% guideline. It is 0.4" narrower.