255/40R20 Tire Size
255/40R20 diameter, sidewall, revolutions per mile, and plus-sizing options.
The 255/40R20 tire size breaks down as: 255 mm section width, 40% aspect ratio, and a 20-inch rim. Below are its real, calculated dimensions.
255/40R20 Calculated Dimensions
| Metric | Value |
|---|---|
| Overall diameter | 28.0 in (712 mm) |
| Sidewall height | 4.0 in (102 mm) |
| Circumference | 88.1 in (2237 mm) |
| Revolutions per mile | 719 |
Typical Fitment
255/40R20 is most often found on luxury sport sedans. Always confirm against the door-jamb placard or owner manual before fitting.
Plus-Sizing Alternatives
These sizes keep the overall diameter close to 255/40R20, so speedometer and gearing stay near stock:
| Size | Diameter | Diameter change |
|---|---|---|
| 265/40R20 | 28.35 in | +1.1% |
| 265/35R21 | 28.30 in | +1.0% |
| 275/30R22 | 28.50 in | +1.7% |
Speedometer Note
If you fit a 265/40R20 instead of 255/40R20, the diameter changes by +1.1%. 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 255/40R20 against any other size and see the exact diameter and speedometer difference.
Frequently Asked Questions
What does 255/40R20 mean?
255/40R20 means a 255 mm wide tire with a 40% 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 255/40R20?
Yes, as long as the replacement keeps the overall diameter close to stock so the speedometer stays accurate. Common plus-size moves from 255/40R20 are 265/40R20, 265/35R21, 275/30R22.
Will 255/40R20 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.