305/30R21 tire size
305/30R21 is a metric size: 28.2" tall, 12.0" wide, on a 21" rim — about a 28x12.00R21. The sidewall is 3.6" and the tire turns 715 times per mile.
305/30R21 specs
| Overall diameter (height) | 28.20" · 716 mm |
|---|---|
| Section width | 12.01" · 305 mm |
| Sidewall height | 3.60" · 92 mm (30% of width) |
| Rim diameter | 21" |
| Circumference | 88.61" · 2.251 m |
| Revolutions per mile | 715 (444 per km) |
| Inch (flotation) equivalent | 28x12.00R21 |
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" | +4.05% | -40 mm | 62.4 mph | compare → |
| 275/35R21 common | 28.58" | +1.33% | -30 mm | 60.8 mph | compare → |
| 295/35R21 | 29.13" | +3.28% | -10 mm | 62.0 mph | compare → |
| 255/40R21 | 29.03" | +2.93% | -50 mm | 61.8 mph | compare → |
| 265/35R21 | 28.30" | +0.35% | -40 mm | 60.2 mph | compare → |
| 245/35R21 | 27.75" | -1.61% | -60 mm | 59.0 mph | compare → |
| 325/30R21 | 28.68" | +1.68% | +20 mm | 61.0 mph | compare → |
| 255/35R21 | 28.03" | -0.63% | -50 mm | 59.6 mph | compare → |
| 315/30R21 | 28.44" | +0.84% | +10 mm | 60.5 mph | compare → |
| 285/35R21 | 28.85" | +2.30% | -20 mm | 61.4 mph | compare → |
| 245/40R21 | 28.72" | +1.81% | -60 mm | 61.1 mph | compare → |
| 295/30R21 | 27.97" | -0.84% | -10 mm | 59.5 mph | compare → |
| 285/30R21 | 27.73" | -1.68% | -20 mm | 59.0 mph | compare → |
| 275/30R21 | 27.50" | -2.51% | -30 mm | 58.5 mph | compare → |
| 255/30R21 | 27.02" | -4.19% | -50 mm | 57.5 mph | calculate → |
| 355/25R21 | 27.99" | -0.77% | +50 mm | 59.5 mph | calculate → |
| 235/45R21 | 29.33" | +3.98% | -70 mm | 62.4 mph | calculate → |
| 295/25R21 | 26.81" | -4.96% | -10 mm | 57.0 mph | calculate → |
| 335/30R21 | 28.91" | +2.51% | +30 mm | 61.5 mph | calculate → |
| 305/35R21 | 29.41" | +4.26% | 0 mm | 62.6 mph | calculate → |
| 265/30R21 | 27.26" | -3.35% | -40 mm | 58.0 mph | calculate → |
| 345/30R21 | 29.15" | +3.35% | +40 mm | 62.0 mph | calculate → |
| 345/25R21 | 27.79" | -1.47% | +40 mm | 59.1 mph | calculate → |
| 225/45R21 | 28.97" | +2.72% | -80 mm | 61.6 mph | calculate → |
| 325/25R21 | 27.40" | -2.86% | +20 mm | 58.3 mph | calculate → |
| 305/25R21 | 27.00" | -4.26% | 0 mm | 57.4 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 | |
|---|---|---|---|---|---|
| 255/40R20 common | 28.03" | -0.61% | -50 mm | 59.6 mph | compare → |
| 295/35R20 | 28.13" | -0.27% | -10 mm | 59.8 mph | compare → |
| 305/35R20 | 28.41" | +0.71% | 0 mm | 60.4 mph | compare → |
| 235/45R20 | 28.33" | +0.43% | -70 mm | 60.3 mph | compare → |
| 265/40R20 | 28.35" | +0.50% | -40 mm | 60.3 mph | compare → |
| 265/30R22 | 28.26" | +0.20% | -40 mm | 60.1 mph | compare → |
| 255/30R22 | 28.02" | -0.64% | -50 mm | 59.6 mph | compare → |
| 345/30R20 | 28.15" | -0.20% | +40 mm | 59.9 mph | calculate → |
| 315/25R22 | 28.20" | -0.01% | +10 mm | 60.0 mph | calculate → |
| 305/25R22 | 28.00" | -0.71% | 0 mm | 59.6 mph | calculate → |
| 225/45R20 | 27.97" | -0.82% | -80 mm | 59.5 mph | calculate → |
| 195/55R20 | 28.44" | +0.85% | -110 mm | 60.5 mph | calculate → |
| 155/65R20 | 27.93" | -0.96% | -150 mm | 59.4 mph | calculate → |
Try another size against 305/30R21
Questions
What size is 305/30R21 in inches?
305/30R21 is about 28.2" tall and 12.0" wide on a 21-inch rim — roughly a 28x12.00R21 in inch sizing.
What does 305/30R21 mean?
305 mm section width, sidewall 30% of the width (3.60"), radial construction, 21-inch rim.
What tire sizes are equivalent to 305/30R21?
On the same 21" rim, within 3% of the diameter: 265/35R21 (+0.35%), 255/35R21 (-0.63%), 355/25R21 (-0.77%), 295/30R21 (-0.84%), 315/30R21 (+0.84%), 275/35R21 (+1.33%).
How many revolutions per mile does a 305/30R21 tire make?
About 715 revolutions per mile (circumference 88.6").
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.