255/50R20 vs 255/55R20 tire size

A 255/55R20 is 1.00 inches (26 mm) taller than a 255/50R20, and the same width. Fit them in place of 255/50R20 tires and the speedometer reads low: at 60 mph on it you’re doing 62.0 mph, 3.3% faster, and the vehicle sits 0.50 inches (13 mm) higher.

Sources checked

Dimension255/50R20255/55R20Change
Overall diameter30.04 in (763 mm)31.04 in (789 mm)+1.00 in (+26 mm)
Section width10.04 in (255 mm)10.04 in (255 mm)0.00 in (0 mm)
Sidewall height5.02 in (127 mm)5.52 in (140 mm)+0.50 in (+13 mm)
Aspect ratio50%55%
Rim diameter20 in20 in
Circumference94.4 in (2,397 mm)97.5 in (2,477 mm)+3.1 in
Revolutions per mile671650−21
Revolutions per km417404−13

Check another speed or size

Metric, LT and flotation

As printed on the sidewall: 265/70R17, LT285/75R16, 33x12.50R15.

Leave it empty to see one size on its own.

mph

Set for your tire size.

Speed unit

At 60 mph on the speedometer you’re doing

62.0mph

+3.3% · the speedometer reads low

Height
+1.00 in (+26 mm)
Width
0.00 in (0 mm)
Ground clearance
+0.50 in (+13 mm)
Each side, same wheel
0.00 in (0 mm)

255/50R20 and 255/55R20 side by side

Dimension255/50R20255/55R20Change
Overall diameter30.04 in (763 mm)31.04 in (789 mm)+1.00 in (+26 mm)
Section width10.04 in (255 mm)10.04 in (255 mm)0.00 in (0 mm)
Sidewall height5.02 in (127 mm)5.52 in (140 mm)+0.50 in (+13 mm)
Aspect ratio50%55%
Rim diameter20 in20 in
Circumference94.4 in97.5 in+3.1 in
Revolutions per mile671650−21
Revolutions per km417404−13

The working

  1. 255/50R20: sidewall = 255 mm × 50% = 127.5 mm = 5.02 in.
  2. Diameter = rim + 2 × sidewall = 20 + 2 × 5.02 = 30.04 in (763 mm).
  3. Circumference = π × 30.04 = 94.4 in; revolutions per mile = 63,360 ÷ 94.4 = 671.
  4. 255/55R20: sidewall = 255 mm × 55% = 140.3 mm = 5.52 in.
  5. Diameter = rim + 2 × sidewall = 20 + 2 × 5.52 = 31.04 in (789 mm).
  6. Circumference = π × 31.04 = 97.5 in; revolutions per mile = 63,360 ÷ 97.5 = 650.
  7. Speedometer. True speed = what it shows × new diameter ÷ old = 60 × 31.04 ÷ 30.04 = 62.0 mph.
  8. Ground clearance. Half the change in diameter: (31.04 − 30.04) ÷ 2 = +0.50 in.

Speedometer

A 255/55R20 turns 650 times a mile against the 255/50R20’s 671. With a speedometer set for 255/50R20, true speed is what it shows × 31.04 ÷ 30.04:

Speedometer showsTrue speedDifference
25 mph25.8 mph+0.8 mph
35 mph36.2 mph+1.2 mph
45 mph46.5 mph+1.5 mph
55 mph56.8 mph+1.8 mph
65 mph67.2 mph+2.2 mph
75 mph77.5 mph+2.5 mph

Clearance

  • Ground clearance: +0.50 in (+13 mm), half the change in diameter.
  • Top of the tread: +0.50 in (+13 mm), so that much less room under the fender.
  • Each side, on the same wheel: no change.

A new wheel with a different width or offset moves the tire again. The sidewall goes from 5.02 to 5.52 inches.

What tire makers publish

Inflated on the maker’s measuring rim; the percentage is against the arithmetic.

TireSize as printedDiameterWidthRevs per mile
Toyo Extensa A/S II255/50R2030.1 in (+0.2%)10.4 in691
Toyo Open Country A/T III255/50R2030.1 in (+0.2%)10.4 in691
Toyo Open Country A/T III255/55R2031 in (−0.1%)10.4 in670

Each size on its own

Other comparisons

Questions

Can I fit 255/55R20 tires instead of 255/50R20?

They fit the same 20-inch wheels. A 255/55R20 is 1.00 inches (26 mm) taller than a 255/50R20, and the same width, so check three things first: that your wheels are within the 255/55R20’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 255/55R20 tires?

With a speedometer set for 255/50R20, the speedometer reads low: at 60 mph on it you’re doing 62.0 mph, 3.3% faster. The odometer is off the same way: over 1,000 miles driven it shows 967.7.

Does 255/55R20 change the ground clearance?

By +0.50 inches (+13 mm), half the change in diameter, at the axles and at the lowest point under them. The width doesn’t change.

Sources

  1. 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)
  2. Toyo Tires: Extensa A/S II sizes and specifications: inflated overall diameter and width, revolutions per mile (checked October 28, 2026)
  3. Toyo Tires: Proxes Sport A/S sizes and specifications: inflated overall diameter and width, revolutions per mile (checked October 28, 2026)
  4. 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)
  5. 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.