315/40R21 Tire Size
315/40R21 diameter, sidewall, revolutions per mile, and plus-sizing options.
The 315/40R21 tire size breaks down as: 315 mm section width, 40% aspect ratio, and a 21-inch rim. Below are its real, calculated dimensions.
315/40R21 Calculated Dimensions
| Metric | Value |
|---|---|
| Overall diameter | 30.9 in (785 mm) |
| Sidewall height | 5.0 in (126 mm) |
| Circumference | 97.1 in (2467 mm) |
| Revolutions per mile | 652 |
Typical Fitment
315/40R21 is most often found on full-size performance SUVs. Always confirm against the door-jamb placard or owner manual before fitting.
Plus-Sizing Alternatives
These sizes keep the overall diameter close to 315/40R21, so speedometer and gearing stay near stock:
| Size | Diameter | Diameter change |
|---|---|---|
| 325/40R21 | 31.24 in | +1.0% |
| 325/35R22 | 30.96 in | +0.1% |
| 335/30R23 | 30.91 in | -0.0% |
Speedometer Note
If you fit a 325/40R21 instead of 315/40R21, the diameter changes by +1.0%. 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 315/40R21 against any other size and see the exact diameter and speedometer difference.
Frequently Asked Questions
What does 315/40R21 mean?
315/40R21 means a 315 mm wide tire with a 40% sidewall aspect ratio mounted on a 21-inch rim. The aspect ratio is the sidewall height as a percentage of the width.
Can I plus-size from 315/40R21?
Yes, as long as the replacement keeps the overall diameter close to stock so the speedometer stays accurate. Common plus-size moves from 315/40R21 are 325/40R21, 325/35R22, 335/30R23.
Will 315/40R21 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.