255/30R21 tire size
255/30R21 is a metric size: 27.0" tall, 10.0" wide, on a 21" rim — about a 27x10.00R21. The sidewall is 3.0" and the tire turns 746 times per mile.
255/30R21 specs
| Overall diameter (height) | 27.02" · 686 mm |
|---|---|
| Section width | 10.04" · 255 mm |
| Sidewall height | 3.01" · 77 mm (30% of width) |
| Rim diameter | 21" |
| Circumference | 84.90" · 2.156 m |
| Revolutions per mile | 746 (464 per km) |
| Inch (flotation) equivalent | 27x10.00R21 |
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 | |
|---|---|---|---|---|---|
| 265/35R21 | 28.30" | +4.73% | +10 mm | 62.8 mph | calculate → |
| 245/35R21 | 27.75" | +2.70% | -10 mm | 61.6 mph | calculate → |
| 255/35R21 | 28.03" | +3.72% | 0 mm | 62.2 mph | calculate → |
| 295/30R21 | 27.97" | +3.50% | +40 mm | 62.1 mph | calculate → |
| 305/30R21 | 28.20" | +4.37% | +50 mm | 62.6 mph | calculate → |
| 285/30R21 | 27.73" | +2.62% | +30 mm | 61.6 mph | calculate → |
| 275/30R21 | 27.50" | +1.75% | +20 mm | 61.0 mph | calculate → |
| 355/25R21 | 27.99" | +3.57% | +100 mm | 62.1 mph | calculate → |
| 295/25R21 | 26.81" | -0.80% | +40 mm | 59.5 mph | calculate → |
| 265/30R21 | 27.26" | +0.87% | +10 mm | 60.5 mph | calculate → |
| 305/25R21 | 27.00" | -0.07% | +50 mm | 60.0 mph | calculate → |
| 325/25R21 | 27.40" | +1.38% | +70 mm | 60.8 mph | calculate → |
| 345/25R21 | 27.79" | +2.84% | +90 mm | 61.7 mph | calculate → |
| 245/30R21 | 26.79" | -0.87% | -10 mm | 59.5 mph | calculate → |
| 275/25R21 | 26.41" | -2.26% | +20 mm | 58.6 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 | |
|---|---|---|---|---|---|
| 255/35R20 common | 27.03" | +0.01% | 0 mm | 60.0 mph | calculate → |
| 295/30R20 | 26.97" | -0.20% | +40 mm | 59.9 mph | calculate → |
| 305/30R20 | 27.20" | +0.67% | +50 mm | 60.4 mph | calculate → |
| 345/25R20 | 26.79" | -0.86% | +90 mm | 59.5 mph | calculate → |
| 225/40R20 | 27.09" | +0.23% | -30 mm | 60.1 mph | calculate → |
| 145/60R20 | 26.85" | -0.64% | -110 mm | 59.6 mph | calculate → |
Try another size against 255/30R21
Questions
What size is 255/30R21 in inches?
255/30R21 is about 27.0" tall and 10.0" wide on a 21-inch rim — roughly a 27x10.00R21 in inch sizing.
What does 255/30R21 mean?
255 mm section width, sidewall 30% of the width (3.01"), radial construction, 21-inch rim.
What tire sizes are equivalent to 255/30R21?
On the same 21" rim, within 3% of the diameter: 305/25R21 (-0.07%), 295/25R21 (-0.80%), 265/30R21 (+0.87%), 245/30R21 (-0.87%), 325/25R21 (+1.38%), 275/30R21 (+1.75%).
How many revolutions per mile does a 255/30R21 tire make?
About 746 revolutions per mile (circumference 84.9").
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.