245/50R20 vs 255/50R20 tire size
A 255/50R20 is 0.39 inches (10 mm) taller and 0.39 inches (10 mm) wider than a 245/50R20. Fit them in place of 245/50R20 tires and the speedometer reads low: at 60 mph on it you’re doing 60.8 mph, 1.3% faster, and the vehicle sits 0.20 inches (5 mm) higher.
Sources checked
| Dimension | 245/50R20 | 255/50R20 | Change |
|---|---|---|---|
| Overall diameter | 29.65 in (753 mm) | 30.04 in (763 mm) | +0.39 in (+10 mm) |
| Section width | 9.65 in (245 mm) | 10.04 in (255 mm) | +0.39 in (+10 mm) |
| Sidewall height | 4.82 in (123 mm) | 5.02 in (127 mm) | +0.20 in (+4 mm) |
| Aspect ratio | 50% | 50% | |
| Rim diameter | 20 in | 20 in | |
| Circumference | 93.1 in (2,366 mm) | 94.4 in (2,397 mm) | +1.3 in |
| Revolutions per mile | 680 | 671 | −9 |
| Revolutions per km | 423 | 417 | −6 |
Check another speed or size
Metric, LT and flotation
At 60 mph on the speedometer you’re doing
60.8mph
+1.3% · the speedometer reads low
- Height
- +0.39 in (+10 mm)
- Width
- +0.39 in (+10 mm)
- Ground clearance
- +0.20 in (+5 mm)
- Each side, same wheel
- +0.20 in (+5 mm)
245/50R20 and 255/50R20 side by side
| Dimension | 245/50R20 | 255/50R20 | Change |
|---|---|---|---|
| Overall diameter | 29.65 in (753 mm) | 30.04 in (763 mm) | +0.39 in (+10 mm) |
| Section width | 9.65 in (245 mm) | 10.04 in (255 mm) | +0.39 in (+10 mm) |
| Sidewall height | 4.82 in (123 mm) | 5.02 in (127 mm) | +0.20 in (+4 mm) |
| Aspect ratio | 50% | 50% | |
| Rim diameter | 20 in | 20 in | |
| Circumference | 93.1 in | 94.4 in | +1.3 in |
| Revolutions per mile | 680 | 671 | −9 |
| Revolutions per km | 423 | 417 | −6 |
The working
- 245/50R20: sidewall = 245 mm × 50% = 122.5 mm = 4.82 in.
- Diameter = rim + 2 × sidewall = 20 + 2 × 4.82 = 29.65 in (753 mm).
- Circumference = π × 29.65 = 93.1 in; revolutions per mile = 63,360 ÷ 93.1 = 680.
- 255/50R20: sidewall = 255 mm × 50% = 127.5 mm = 5.02 in.
- Diameter = rim + 2 × sidewall = 20 + 2 × 5.02 = 30.04 in (763 mm).
- Circumference = π × 30.04 = 94.4 in; revolutions per mile = 63,360 ÷ 94.4 = 671.
- Speedometer. True speed = what it shows × new diameter ÷ old = 60 × 30.04 ÷ 29.65 = 60.8 mph.
- Ground clearance. Half the change in diameter: (30.04 − 29.65) ÷ 2 = +0.20 in.
Speedometer
A 255/50R20 turns 671 times a mile against the 245/50R20’s 680. With a speedometer set for 245/50R20, true speed is what it shows × 30.04 ÷ 29.65:
| Speedometer shows | True speed | Difference |
|---|---|---|
| 25 mph | 25.3 mph | +0.3 mph |
| 35 mph | 35.5 mph | +0.5 mph |
| 45 mph | 45.6 mph | +0.6 mph |
| 55 mph | 55.7 mph | +0.7 mph |
| 65 mph | 65.9 mph | +0.9 mph |
| 75 mph | 76.0 mph | +1.0 mph |
Clearance
- Ground clearance: +0.20 in (+5 mm), half the change in diameter.
- Top of the tread: +0.20 in (+5 mm), so that much less room under the fender.
- Each side, on the same wheel: +0.20 in (+5 mm), toward the suspension inside and toward the fender lip outside.
A new wheel with a different width or offset moves the tire again. The sidewall goes from 4.82 to 5.02 inches.
What tire makers publish
Inflated on the maker’s measuring rim; the percentage is against the arithmetic.
| Tire | Size as printed | Diameter | Width | Revs per mile |
|---|---|---|---|---|
| Toyo Open Country A/T III | 245/50R20 | 29.7 in (+0.2%) | 10 in | 700 |
| Toyo Extensa A/S II | 255/50R20 | 30.1 in (+0.2%) | 10.4 in | 691 |
| Toyo Open Country A/T III | 255/50R20 | 30.1 in (+0.2%) | 10.4 in | 691 |
Each size on its own
- 245/50R20 tire size: 29.6 × 9.6 in
- 255/50R20 tire size: 30.0 × 10.0 in
Other comparisons
Questions
Can I fit 255/50R20 tires instead of 245/50R20?
They fit the same 20-inch wheels. A 255/50R20 is 0.39 inches (10 mm) taller and 0.39 inches (10 mm) wider than a 245/50R20, so check three things first: that your wheels are within the 255/50R20’s approved rim width range, which tire makers list for each size; that it clears the fenders and suspension at full lock and full travel; and that its load index meets what your door-jamb placard asks for.
How far off is the speedometer with 255/50R20 tires?
With a speedometer set for 245/50R20, the speedometer reads low: at 60 mph on it you’re doing 60.8 mph, 1.3% faster. The odometer is off the same way: over 1,000 miles driven it shows 986.9.
Does 255/50R20 change the ground clearance?
By +0.20 inches (+5 mm), half the change in diameter, at the axles and at the lowest point under them. On the same wheel, each sidewall sits 0.20 inches further out, inside and outside.
Sources
- Bridgestone Americas: Determining tire size: P-metric, Euro-metric, LT-metric, C-type and flotation sizes, and what each number means (checked October 28, 2026)
- Toyo Tires: Extensa A/S II sizes and specifications: inflated overall diameter and width, revolutions per mile (checked October 28, 2026)
- Toyo Tires: Proxes Sport A/S sizes and specifications: inflated overall diameter and width, revolutions per mile (checked October 28, 2026)
- Toyo Tires: Open Country A/T III sizes and specifications: P-metric, LT-metric and flotation sizes, with revolutions per mile (checked October 28, 2026)
- Nitto Tire: Trail Grappler M/T sizes and specs: inflated overall diameter and width, LT-metric and flotation sizes (checked October 28, 2026)
Spotted a number that doesn’t match your manual or a source? Tell us; corrections are logged on the sources page.