315/40R21 tire size
315/40R21 is a metric size: 30.9" tall, 12.4" wide, on a 21" rim — about a 31x12.50R21. The sidewall is 5.0" and the tire turns 652 times per mile.
315/40R21 specs
| Overall diameter (height) | 30.92" · 785 mm |
|---|---|
| Section width | 12.40" · 315 mm |
| Sidewall height | 4.96" · 126 mm (40% of width) |
| Rim diameter | 21" |
| Circumference | 97.14" · 2.467 m |
| Revolutions per mile | 652 (405 per km) |
| Inch (flotation) equivalent | 31x12.50R21 |
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" | -0.57% | -40 mm | 59.7 mph | compare → |
| 285/45R21 | 31.10" | +0.57% | -30 mm | 60.3 mph | compare → |
| 295/40R21 | 30.29" | -2.04% | -20 mm | 58.8 mph | compare → |
| 265/45R21 | 30.39" | -1.72% | -50 mm | 59.0 mph | compare → |
| 285/40R21 | 29.98" | -3.06% | -30 mm | 58.2 mph | compare → |
| 275/40R21 | 29.66" | -4.07% | -40 mm | 57.6 mph | compare → |
| 315/35R21 | 29.68" | -4.01% | 0 mm | 57.6 mph | compare → |
| 255/45R21 | 30.04" | -2.86% | -60 mm | 58.3 mph | compare → |
| 245/45R21 | 29.68" | -4.01% | -70 mm | 57.6 mph | compare → |
| 255/50R21 | 31.04" | +0.38% | -60 mm | 60.2 mph | compare → |
| 275/50R21 | 31.83" | +2.93% | -40 mm | 61.8 mph | compare → |
| 235/50R21 | 30.25" | -2.16% | -80 mm | 58.7 mph | calculate → |
| 305/35R21 | 29.41" | -4.90% | -10 mm | 57.1 mph | calculate → |
| 305/40R21 | 30.61" | -1.02% | -10 mm | 59.4 mph | calculate → |
| 325/40R21 | 31.24" | +1.02% | +10 mm | 60.6 mph | calculate → |
| 315/45R21 | 32.16" | +4.01% | 0 mm | 62.4 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" | +0.39% | -60 mm | 60.2 mph | compare → |
| 275/50R20 common | 30.83" | -0.31% | -40 mm | 59.8 mph | compare → |
| 275/40R22 | 30.66" | -0.84% | -40 mm | 59.5 mph | compare → |
| 285/40R22 | 30.98" | +0.18% | -30 mm | 60.1 mph | compare → |
| 285/50R20 | 31.22" | +0.97% | -30 mm | 60.6 mph | compare → |
| 325/35R22 | 30.96" | +0.11% | +10 mm | 60.1 mph | compare → |
| 315/35R22 | 30.68" | -0.78% | 0 mm | 59.5 mph | compare → |
| 255/45R22 | 31.04" | +0.37% | -60 mm | 60.2 mph | calculate → |
| 235/60R20 | 31.10" | +0.59% | -80 mm | 60.4 mph | calculate → |
| 31x18.00R20 | 31.00" | +0.25% | +142 mm | 60.2 mph | calculate → |
| 305/45R20 | 30.81" | -0.37% | -10 mm | 59.8 mph | calculate → |
Try another size against 315/40R21
Questions
What size is 315/40R21 in inches?
315/40R21 is about 30.9" tall and 12.4" wide on a 21-inch rim — roughly a 31x12.50R21 in inch sizing.
What does 315/40R21 mean?
315 mm section width, sidewall 40% of the width (4.96"), radial construction, 21-inch rim.
What tire sizes are equivalent to 315/40R21?
On the same 21" rim, within 3% of the diameter: 255/50R21 (+0.38%), 275/45R21 (-0.57%), 285/45R21 (+0.57%), 305/40R21 (-1.02%), 325/40R21 (+1.02%), 265/45R21 (-1.72%).
How many revolutions per mile does a 315/40R21 tire make?
About 652 revolutions per mile (circumference 97.1").
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.