325/40R21 tire size
325/40R21 is a metric size: 31.2" tall, 12.8" wide, on a 21" rim — about a 31x13.00R21. The sidewall is 5.1" and the tire turns 646 times per mile.
325/40R21 specs
| Overall diameter (height) | 31.24" · 793 mm |
|---|---|
| Section width | 12.80" · 325 mm |
| Sidewall height | 5.12" · 130 mm (40% of width) |
| Rim diameter | 21" |
| Circumference | 98.13" · 2.493 m |
| Revolutions per mile | 646 (401 per km) |
| Inch (flotation) equivalent | 31x13.00R21 |
Equivalent sizes on a 21" rim
Sizes within ±5% of the overall diameter. Green rows are inside the usual ±3% limit for keeping the speedometer, ABS and gearing right; red rows are bigger changes that need clearance checks.
| Size | Diameter | Difference | Width | Speedo at 60 | |
|---|---|---|---|---|---|
| 275/45R21 | 30.74" | -1.58% | -50 mm | 59.1 mph | calculate → |
| 285/45R21 | 31.10" | -0.44% | -40 mm | 59.7 mph | calculate → |
| 295/40R21 | 30.29" | -3.02% | -30 mm | 58.2 mph | calculate → |
| 265/45R21 | 30.39" | -2.71% | -60 mm | 58.4 mph | calculate → |
| 285/40R21 | 29.98" | -4.03% | -40 mm | 57.6 mph | calculate → |
| 315/35R21 | 29.68" | -4.98% | -10 mm | 57.0 mph | calculate → |
| 315/40R21 | 30.92" | -1.01% | -10 mm | 59.4 mph | calculate → |
| 255/45R21 | 30.04" | -3.84% | -70 mm | 57.7 mph | calculate → |
| 245/45R21 | 29.68" | -4.98% | -80 mm | 57.0 mph | calculate → |
| 255/50R21 | 31.04" | -0.63% | -70 mm | 59.6 mph | calculate → |
| 275/50R21 | 31.83" | +1.89% | -50 mm | 61.1 mph | calculate → |
| 235/50R21 | 30.25" | -3.15% | -90 mm | 58.1 mph | calculate → |
| 305/40R21 | 30.61" | -2.02% | -20 mm | 58.8 mph | calculate → |
| 315/45R21 | 32.16" | +2.96% | -10 mm | 61.8 mph | calculate → |
Plus / minus sizing: 20" and 22" rims
Same overall height on a different rim. Check wheel width and clearance before switching.
| Size | Diameter | Difference | Width | Speedo at 60 | |
|---|---|---|---|---|---|
| 255/55R20 common | 31.04" | -0.62% | -70 mm | 59.6 mph | calculate → |
| 285/40R22 | 30.98" | -0.83% | -40 mm | 59.5 mph | calculate → |
| 285/50R20 | 31.22" | -0.05% | -40 mm | 60.0 mph | calculate → |
| 325/35R22 | 30.96" | -0.89% | 0 mm | 59.5 mph | calculate → |
| 295/40R22 | 31.29" | +0.18% | -30 mm | 60.1 mph | calculate → |
| 255/45R22 | 31.04" | -0.64% | -70 mm | 59.6 mph | calculate → |
| 235/60R20 | 31.10" | -0.43% | -90 mm | 59.7 mph | calculate → |
| 31x18.00R20 | 31.00" | -0.76% | +132 mm | 59.5 mph | calculate → |
| 265/55R20 | 31.48" | +0.77% | -60 mm | 60.5 mph | calculate → |
| 265/45R22 | 31.39" | +0.49% | -60 mm | 60.3 mph | calculate → |
Try another size against 325/40R21
Questions
What size is 325/40R21 in inches?
325/40R21 is about 31.2" tall and 12.8" wide on a 21-inch rim — roughly a 31x13.00R21 in inch sizing.
What does 325/40R21 mean?
325 mm section width, sidewall 40% of the width (5.12"), radial construction, 21-inch rim.
What tire sizes are equivalent to 325/40R21?
On the same 21" rim, within 3% of the diameter: 285/45R21 (-0.44%), 255/50R21 (-0.63%), 315/40R21 (-1.01%), 275/45R21 (-1.58%), 275/50R21 (+1.89%), 305/40R21 (-2.02%).
How many revolutions per mile does a 325/40R21 tire make?
About 646 revolutions per mile (circumference 98.1").
How these numbers are computed
Diameter = rim + 2 × sidewall, sidewall = width × aspect ÷ 100, circumference = π × diameter, revolutions per mile = 63,360 ÷ circumference. Real tires differ by brand by a few tenths of an inch. Last reviewed October 14, 2026.