305/30R20 Tire Size
305/30R20 diameter, sidewall, revolutions per mile, and plus-sizing options.
The 305/30R20 tire size breaks down as: 305 mm section width, 30% aspect ratio, and a 20-inch rim. Below are its real, calculated dimensions.
305/30R20 Calculated Dimensions
| Metric | Value |
|---|---|
| Overall diameter | 27.2 in (691 mm) |
| Sidewall height | 3.6 in (92 mm) |
| Circumference | 85.5 in (2171 mm) |
| Revolutions per mile | 741 |
Typical Fitment
305/30R20 is most often found on performance GT cars. Always confirm against the door-jamb placard or owner manual before fitting.
Plus-Sizing Alternatives
These sizes keep the overall diameter close to 305/30R20, so speedometer and gearing stay near stock:
| Size | Diameter | Diameter change |
|---|---|---|
| 315/30R20 | 27.44 in | +0.9% |
| 315/25R21 | 27.20 in | -0.0% |
| 325/20R22 | 27.12 in | -0.3% |
Speedometer Note
If you fit a 315/30R20 instead of 305/30R20, the diameter changes by +0.9%. A taller tire makes your speedometer read low — at a true 60 mph it would show roughly 59 mph. A shorter tire has the opposite effect, so the gauge reads high.
Use the tire calculator to compare 305/30R20 against any other size and see the exact diameter and speedometer difference.
Frequently Asked Questions
What does 305/30R20 mean?
305/30R20 means a 305 mm wide tire with a 30% sidewall aspect ratio mounted on a 20-inch rim. The aspect ratio is the sidewall height as a percentage of the width.
Can I plus-size from 305/30R20?
Yes, as long as the replacement keeps the overall diameter close to stock so the speedometer stays accurate. Common plus-size moves from 305/30R20 are 315/30R20, 315/25R21, 325/20R22.
Will 305/30R20 affect my speedometer?
Only if it differs in diameter from your original size. A taller tire reads low; a shorter tire reads high. Compare the two sizes with the calculator to find the percentage.