255/45R21 tire size
255/45R21 is a metric size: 30.0" tall, 10.0" wide, on a 21" rim — about a 30x10.00R21. The sidewall is 4.5" and the tire turns 671 times per mile.
255/45R21 specs
| Overall diameter (height) | 30.04" · 763 mm |
|---|---|
| Section width | 10.04" · 255 mm |
| Sidewall height | 4.52" · 115 mm (45% of width) |
| Rim diameter | 21" |
| Circumference | 94.36" · 2.397 m |
| Revolutions per mile | 671 (417 per km) |
| Inch (flotation) equivalent | 30x10.00R21 |
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" | -2.29% | +10 mm | 58.6 mph | compare → |
| 275/35R21 common | 28.58" | -4.85% | +20 mm | 57.1 mph | compare → |
| 295/35R21 | 29.13" | -3.01% | +40 mm | 58.2 mph | compare → |
| 275/45R21 | 30.74" | +2.36% | +20 mm | 61.4 mph | compare → |
| 255/40R21 | 29.03" | -3.34% | 0 mm | 58.0 mph | compare → |
| 295/40R21 | 30.29" | +0.85% | +40 mm | 60.5 mph | compare → |
| 285/45R21 | 31.10" | +3.54% | +30 mm | 62.1 mph | compare → |
| 285/40R21 | 29.98" | -0.20% | +30 mm | 59.9 mph | compare → |
| 265/45R21 | 30.39" | +1.18% | +10 mm | 60.7 mph | compare → |
| 325/30R21 | 28.68" | -4.52% | +70 mm | 57.3 mph | compare → |
| 275/40R21 | 29.66" | -1.25% | +20 mm | 59.3 mph | compare → |
| 285/35R21 | 28.85" | -3.93% | +30 mm | 57.6 mph | compare → |
| 315/35R21 | 29.68" | -1.18% | +60 mm | 59.3 mph | compare → |
| 245/40R21 | 28.72" | -4.39% | -10 mm | 57.4 mph | compare → |
| 315/40R21 | 30.92" | +2.95% | +60 mm | 61.8 mph | compare → |
| 245/45R21 | 29.68" | -1.18% | -10 mm | 59.3 mph | compare → |
| 255/50R21 | 31.04" | +3.34% | 0 mm | 62.0 mph | compare → |
| 235/50R21 | 30.25" | +0.72% | -20 mm | 60.4 mph | calculate → |
| 235/45R21 | 29.33" | -2.36% | -20 mm | 58.6 mph | calculate → |
| 305/35R21 | 29.41" | -2.10% | +50 mm | 58.7 mph | calculate → |
| 335/30R21 | 28.91" | -3.74% | +80 mm | 57.8 mph | calculate → |
| 345/30R21 | 29.15" | -2.95% | +90 mm | 58.2 mph | calculate → |
| 225/45R21 | 28.97" | -3.54% | -30 mm | 57.9 mph | calculate → |
| 305/40R21 | 30.61" | +1.90% | +50 mm | 61.1 mph | calculate → |
| 325/40R21 | 31.24" | +4.00% | +70 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 | |
|---|---|---|---|---|---|
| 255/50R20 common | 30.04" | +0.01% | 0 mm | 60.0 mph | compare → |
| 275/45R20 common | 29.74" | -0.97% | +20 mm | 59.4 mph | compare → |
| 235/55R20 common | 30.18" | +0.47% | -20 mm | 60.3 mph | compare → |
| 285/35R22 | 29.85" | -0.60% | +30 mm | 59.6 mph | compare → |
| 285/45R20 | 30.10" | +0.21% | +30 mm | 60.1 mph | compare → |
| 295/35R22 | 30.13" | +0.31% | +40 mm | 60.2 mph | compare → |
| 255/40R22 | 30.03" | -0.01% | 0 mm | 60.0 mph | compare → |
Try another size against 255/45R21
Questions
What size is 255/45R21 in inches?
255/45R21 is about 30.0" tall and 10.0" wide on a 21-inch rim — roughly a 30x10.00R21 in inch sizing.
What does 255/45R21 mean?
255 mm section width, sidewall 45% of the width (4.52"), radial construction, 21-inch rim.
What tire sizes are equivalent to 255/45R21?
On the same 21" rim, within 3% of the diameter: 285/40R21 (-0.20%), 235/50R21 (+0.72%), 295/40R21 (+0.85%), 245/45R21 (-1.18%), 315/35R21 (-1.18%), 265/45R21 (+1.18%).
How many revolutions per mile does a 255/45R21 tire make?
About 671 revolutions per mile (circumference 94.4").
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.