305/40R20 tire size
305/40R20 is a metric size: 29.6" tall, 12.0" wide, on a 20" rim — about a 29.5x12.00R20. The sidewall is 4.8" and the tire turns 681 times per mile.
305/40R20 specs
| Overall diameter (height) | 29.61" · 752 mm |
|---|---|
| Section width | 12.01" · 305 mm |
| Sidewall height | 4.80" · 122 mm (40% of width) |
| Rim diameter | 20" |
| Circumference | 93.01" · 2.362 m |
| Revolutions per mile | 681 (423 per km) |
| Inch (flotation) equivalent | 29.5x12.00R20 |
Equivalent sizes on a 20" 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 | |
|---|---|---|---|---|---|
| 245/45R20 common | 28.68" | -3.12% | -60 mm | 58.1 mph | compare → |
| 255/45R20 common | 29.04" | -1.93% | -50 mm | 58.8 mph | compare → |
| 255/50R20 common | 30.04" | +1.46% | -50 mm | 60.9 mph | compare → |
| 275/40R20 common | 28.66" | -3.19% | -30 mm | 58.1 mph | compare → |
| 265/50R20 common | 30.43" | +2.79% | -40 mm | 61.7 mph | compare → |
| 245/50R20 common | 29.65" | +0.13% | -60 mm | 60.1 mph | compare → |
| 275/45R20 common | 29.74" | +0.47% | -30 mm | 60.3 mph | compare → |
| 255/55R20 common | 31.04" | +4.85% | -50 mm | 62.9 mph | compare → |
| 235/55R20 common | 30.18" | +1.93% | -70 mm | 61.2 mph | compare → |
| 265/45R20 common | 29.39" | -0.73% | -40 mm | 59.6 mph | compare → |
| 315/35R20 common | 28.68" | -3.12% | +10 mm | 58.1 mph | compare → |
| 275/50R20 common | 30.83" | +4.12% | -30 mm | 62.5 mph | compare → |
| 295/40R20 | 29.29" | -1.06% | -10 mm | 59.4 mph | compare → |
| 295/35R20 | 28.13" | -4.99% | -10 mm | 57.0 mph | compare → |
| 305/35R20 | 28.41" | -4.06% | 0 mm | 57.6 mph | compare → |
| 285/40R20 | 28.98" | -2.13% | -20 mm | 58.7 mph | compare → |
| 235/50R20 | 29.25" | -1.20% | -70 mm | 59.3 mph | compare → |
| 285/45R20 | 30.10" | +1.66% | -20 mm | 61.0 mph | compare → |
| 235/45R20 | 28.33" | -4.32% | -70 mm | 57.4 mph | compare → |
| 295/45R20 | 30.45" | +2.86% | -10 mm | 61.7 mph | compare → |
| 265/40R20 | 28.35" | -4.26% | -40 mm | 57.4 mph | compare → |
| 345/30R20 | 28.15" | -4.92% | +40 mm | 57.0 mph | calculate → |
| 325/35R20 | 28.96" | -2.19% | +20 mm | 58.7 mph | calculate → |
| 29x15.00R20 | 29.00" | -2.05% | +76 mm | 58.8 mph | calculate → |
| 29x18.00R20 | 29.00" | -2.05% | +152 mm | 58.8 mph | calculate → |
| 305/45R20 | 30.81" | +4.06% | 0 mm | 62.4 mph | calculate → |
| 31x18.00R20 | 31.00" | +4.71% | +152 mm | 62.8 mph | calculate → |
| 175/70R20 | 29.65" | +0.13% | -130 mm | 60.1 mph | calculate → |
| 195/55R20 | 28.44" | -3.92% | -110 mm | 57.6 mph | calculate → |
Plus / minus sizing: 19" and 21" rims
Same overall height on a different rim. Check wheel width and clearance before switching.
| Size | Diameter | Difference | Width | Speedo at 60 | |
|---|---|---|---|---|---|
| 245/55R19 common | 29.61" | +0.01% | -60 mm | 60.0 mph | compare → |
| 265/50R19 common | 29.43" | -0.59% | -40 mm | 59.6 mph | compare → |
| 265/40R21 common | 29.35" | -0.88% | -40 mm | 59.5 mph | compare → |
| 275/40R21 | 29.66" | +0.19% | -30 mm | 60.1 mph | compare → |
| 315/35R21 | 29.68" | +0.25% | +10 mm | 60.2 mph | compare → |
| 245/45R21 | 29.68" | +0.25% | -60 mm | 60.2 mph | compare → |
| 275/50R19 | 29.83" | +0.74% | -30 mm | 60.4 mph | compare → |
| 235/45R21 | 29.33" | -0.94% | -70 mm | 59.4 mph | calculate → |
| 295/45R19 | 29.45" | -0.52% | -10 mm | 59.7 mph | calculate → |
| 305/35R21 | 29.41" | -0.68% | 0 mm | 59.6 mph | calculate → |
Try another size against 305/40R20
Questions
What size is 305/40R20 in inches?
305/40R20 is about 29.6" tall and 12.0" wide on a 20-inch rim — roughly a 29.5x12.00R20 in inch sizing.
What does 305/40R20 mean?
305 mm section width, sidewall 40% of the width (4.80"), radial construction, 20-inch rim.
What tire sizes are equivalent to 305/40R20?
On the same 20" rim, within 3% of the diameter: 175/70R20 (+0.13%), 245/50R20 (+0.13%), 275/45R20 (+0.47%), 265/45R20 (-0.73%), 295/40R20 (-1.06%), 235/50R20 (-1.20%).
How many revolutions per mile does a 305/40R20 tire make?
About 681 revolutions per mile (circumference 93.0").
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.