255/40R21 tire size
255/40R21 is a metric size: 29.0" tall, 10.0" wide, on a 21" rim — about a 29x10.00R21. The sidewall is 4.0" and the tire turns 695 times per mile.
255/40R21 specs
| Overall diameter (height) | 29.03" · 737 mm |
|---|---|
| Section width | 10.04" · 255 mm |
| Sidewall height | 4.02" · 102 mm (40% of width) |
| Rim diameter | 21" |
| Circumference | 91.21" · 2.317 m |
| Revolutions per mile | 695 (432 per km) |
| Inch (flotation) equivalent | 29x10.00R21 |
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" | +1.08% | +10 mm | 60.7 mph | compare → |
| 275/35R21 common | 28.58" | -1.56% | +20 mm | 59.1 mph | compare → |
| 295/35R21 | 29.13" | +0.34% | +40 mm | 60.2 mph | compare → |
| 265/35R21 | 28.30" | -2.51% | +10 mm | 58.5 mph | compare → |
| 295/40R21 | 30.29" | +4.34% | +40 mm | 62.6 mph | compare → |
| 245/35R21 | 27.75" | -4.41% | -10 mm | 57.4 mph | compare → |
| 285/40R21 | 29.98" | +3.25% | +30 mm | 62.0 mph | compare → |
| 265/45R21 | 30.39" | +4.68% | +10 mm | 62.8 mph | compare → |
| 325/30R21 | 28.68" | -1.22% | +70 mm | 59.3 mph | compare → |
| 275/40R21 | 29.66" | +2.17% | +20 mm | 61.3 mph | compare → |
| 255/35R21 | 28.03" | -3.46% | 0 mm | 57.9 mph | compare → |
| 285/35R21 | 28.85" | -0.61% | +30 mm | 59.6 mph | compare → |
| 315/30R21 | 28.44" | -2.03% | +60 mm | 58.8 mph | compare → |
| 245/40R21 | 28.72" | -1.08% | -10 mm | 59.3 mph | compare → |
| 315/35R21 | 29.68" | +2.24% | +60 mm | 61.3 mph | compare → |
| 305/30R21 | 28.20" | -2.85% | +50 mm | 58.3 mph | compare → |
| 295/30R21 | 27.97" | -3.66% | +40 mm | 57.8 mph | compare → |
| 255/45R21 | 30.04" | +3.46% | 0 mm | 62.1 mph | compare → |
| 285/30R21 | 27.73" | -4.48% | +30 mm | 57.3 mph | compare → |
| 245/45R21 | 29.68" | +2.24% | -10 mm | 61.3 mph | compare → |
| 235/50R21 | 30.25" | +4.20% | -20 mm | 62.5 mph | calculate → |
| 355/25R21 | 27.99" | -3.59% | +100 mm | 57.8 mph | calculate → |
| 235/45R21 | 29.33" | +1.02% | -20 mm | 60.6 mph | calculate → |
| 335/30R21 | 28.91" | -0.41% | +80 mm | 59.8 mph | calculate → |
| 305/35R21 | 29.41" | +1.29% | +50 mm | 60.8 mph | calculate → |
| 345/30R21 | 29.15" | +0.41% | +90 mm | 60.2 mph | calculate → |
| 225/45R21 | 28.97" | -0.20% | -30 mm | 59.9 mph | calculate → |
| 345/25R21 | 27.79" | -4.27% | +90 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 | |
|---|---|---|---|---|---|
| 255/45R20 common | 29.04" | +0.01% | 0 mm | 60.0 mph | compare → |
| 265/35R22 | 29.30" | +0.94% | +10 mm | 60.6 mph | compare → |
| 295/40R20 | 29.29" | +0.90% | +40 mm | 60.5 mph | compare → |
| 285/40R20 | 28.98" | -0.19% | +30 mm | 59.9 mph | compare → |
| 235/50R20 | 29.25" | +0.76% | -20 mm | 60.5 mph | compare → |
| 295/30R22 | 28.97" | -0.22% | +40 mm | 59.9 mph | compare → |
| 255/35R22 | 29.03" | -0.01% | 0 mm | 60.0 mph | calculate → |
| 325/35R20 | 28.96" | -0.26% | +70 mm | 59.8 mph | calculate → |
| 29x15.00R20 | 29.00" | -0.11% | +126 mm | 59.9 mph | calculate → |
| 29x18.00R20 | 29.00" | -0.11% | +202 mm | 59.9 mph | calculate → |
| 355/25R22 | 28.99" | -0.15% | +100 mm | 59.9 mph | calculate → |
| 305/30R22 | 29.20" | +0.60% | +50 mm | 60.4 mph | calculate → |
Try another size against 255/40R21
Questions
What size is 255/40R21 in inches?
255/40R21 is about 29.0" tall and 10.0" wide on a 21-inch rim — roughly a 29x10.00R21 in inch sizing.
What does 255/40R21 mean?
255 mm section width, sidewall 40% of the width (4.02"), radial construction, 21-inch rim.
What tire sizes are equivalent to 255/40R21?
On the same 21" rim, within 3% of the diameter: 225/45R21 (-0.20%), 295/35R21 (+0.34%), 335/30R21 (-0.41%), 345/30R21 (+0.41%), 285/35R21 (-0.61%), 235/45R21 (+1.02%).
How many revolutions per mile does a 255/40R21 tire make?
About 695 revolutions per mile (circumference 91.2").
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.