305/40R21 tire size
305/40R21 is a metric size: 30.6" tall, 12.0" wide, on a 21" rim — about a 30.5x12.00R21. The sidewall is 4.8" and the tire turns 659 times per mile.
305/40R21 specs
| Overall diameter (height) | 30.61" · 777 mm |
|---|---|
| Section width | 12.01" · 305 mm |
| Sidewall height | 4.80" · 122 mm (40% of width) |
| Rim diameter | 21" |
| Circumference | 96.15" · 2.442 m |
| Revolutions per mile | 659 (409 per km) |
| Inch (flotation) equivalent | 30.5x12.00R21 |
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 | |
|---|---|---|---|---|---|
| 265/40R21 common | 29.35" | -4.12% | -40 mm | 57.5 mph | calculate → |
| 295/35R21 | 29.13" | -4.82% | -10 mm | 57.1 mph | calculate → |
| 275/45R21 | 30.74" | +0.45% | -30 mm | 60.3 mph | calculate → |
| 295/40R21 | 30.29" | -1.03% | -10 mm | 59.4 mph | calculate → |
| 285/45R21 | 31.10" | +1.61% | -20 mm | 61.0 mph | calculate → |
| 265/45R21 | 30.39" | -0.71% | -40 mm | 59.6 mph | calculate → |
| 285/40R21 | 29.98" | -2.06% | -20 mm | 58.8 mph | calculate → |
| 275/40R21 | 29.66" | -3.09% | -30 mm | 58.1 mph | calculate → |
| 315/35R21 | 29.68" | -3.02% | +10 mm | 58.2 mph | calculate → |
| 315/40R21 | 30.92" | +1.03% | +10 mm | 60.6 mph | calculate → |
| 255/45R21 | 30.04" | -1.87% | -50 mm | 58.9 mph | calculate → |
| 245/45R21 | 29.68" | -3.02% | -60 mm | 58.2 mph | calculate → |
| 255/50R21 | 31.04" | +1.41% | -50 mm | 60.8 mph | calculate → |
| 275/50R21 | 31.83" | +3.99% | -30 mm | 62.4 mph | calculate → |
| 235/50R21 | 30.25" | -1.16% | -70 mm | 59.3 mph | calculate → |
| 235/45R21 | 29.33" | -4.18% | -70 mm | 57.5 mph | calculate → |
| 305/35R21 | 29.41" | -3.92% | 0 mm | 57.6 mph | calculate → |
| 345/30R21 | 29.15" | -4.76% | +40 mm | 57.1 mph | calculate → |
| 325/40R21 | 31.24" | +2.06% | +20 mm | 61.2 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 | |
|---|---|---|---|---|---|
| 265/50R20 common | 30.43" | -0.57% | -40 mm | 59.7 mph | calculate → |
| 275/50R20 common | 30.83" | +0.72% | -30 mm | 60.4 mph | calculate → |
| 275/40R22 | 30.66" | +0.18% | -30 mm | 60.1 mph | calculate → |
| 265/40R22 | 30.35" | -0.85% | -40 mm | 59.5 mph | calculate → |
| 295/45R20 | 30.45" | -0.50% | -10 mm | 59.7 mph | calculate → |
| 315/35R22 | 30.68" | +0.24% | +10 mm | 60.1 mph | calculate → |
| 305/35R22 | 30.41" | -0.66% | 0 mm | 59.6 mph | calculate → |
| 305/45R20 | 30.81" | +0.66% | 0 mm | 60.4 mph | calculate → |
Try another size against 305/40R21
Questions
What size is 305/40R21 in inches?
305/40R21 is about 30.6" tall and 12.0" wide on a 21-inch rim — roughly a 30.5x12.00R21 in inch sizing.
What does 305/40R21 mean?
305 mm section width, sidewall 40% of the width (4.80"), radial construction, 21-inch rim.
What tire sizes are equivalent to 305/40R21?
On the same 21" rim, within 3% of the diameter: 275/45R21 (+0.45%), 265/45R21 (-0.71%), 295/40R21 (-1.03%), 315/40R21 (+1.03%), 235/50R21 (-1.16%), 255/50R21 (+1.41%).
How many revolutions per mile does a 305/40R21 tire make?
About 659 revolutions per mile (circumference 96.2").
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.