235/40R18 Tire Size
235/40R18 diameter, sidewall, revolutions per mile, and plus-sizing options.
The 235/40R18 tire size breaks down as: 235 mm section width, 40% aspect ratio, and a 18-inch rim. Below are its real, calculated dimensions.
235/40R18 Calculated Dimensions
| Metric | Value |
|---|---|
| Overall diameter | 25.4 in (645 mm) |
| Sidewall height | 3.7 in (94 mm) |
| Circumference | 79.8 in (2027 mm) |
| Revolutions per mile | 794 |
Typical Fitment
235/40R18 is most often found on 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 235/40R18, so speedometer and gearing stay near stock:
| Size | Diameter | Diameter change |
|---|---|---|
| 245/40R18 | 25.72 in | +1.2% |
| 245/35R19 | 25.75 in | +1.4% |
| 255/25R20 | 25.02 in | -1.5% |
Speedometer Note
If you fit a 245/40R18 instead of 235/40R18, the diameter changes by +1.2%. 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 235/40R18 against any other size and see the exact diameter and speedometer difference.
Frequently Asked Questions
What does 235/40R18 mean?
235/40R18 means a 235 mm wide tire with a 40% sidewall aspect ratio mounted on a 18-inch rim. The aspect ratio is the sidewall height as a percentage of the width.
Can I plus-size from 235/40R18?
Yes, as long as the replacement keeps the overall diameter close to stock so the speedometer stays accurate. Common plus-size moves from 235/40R18 are 245/40R18, 245/35R19, 255/25R20.
Will 235/40R18 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.