255/55R20 tire size

A 255/55R20 tire is 31.0 inches (789 mm) tall and 10.0 inches (255 mm) wide, for a 20-inch wheel. Each sidewall is 5.5 inches (140 mm), 55% of the width, and the tire makes about 650 revolutions a mile.

Sources checked

Dimension255/55R20
Overall diameter31.04 in (789 mm)
Section width10.04 in (255 mm)
Sidewall height5.52 in (140 mm)
Aspect ratio55%
Rim diameter20 in
Circumference97.5 in (2,477 mm)
Revolutions per mile650
Revolutions per km404

Compare 255/55R20 with another 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

58.1mph

−3.2% · the speedometer reads high

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)

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

Dimension255/55R20255/50R20Change
Overall diameter31.04 in (789 mm)30.04 in (763 mm)−1.00 in (−26 mm)
Section width10.04 in (255 mm)10.04 in (255 mm)0.00 in (0 mm)
Sidewall height5.52 in (140 mm)5.02 in (127 mm)−0.50 in (−13 mm)
Aspect ratio55%50%
Rim diameter20 in20 in
Circumference97.5 in94.4 in−3.1 in
Revolutions per mile650671+21
Revolutions per km404417+13

The working

  1. 255/55R20: sidewall = 255 mm × 55% = 140.3 mm = 5.52 in.
  2. Diameter = rim + 2 × sidewall = 20 + 2 × 5.52 = 31.04 in (789 mm).
  3. Circumference = π × 31.04 = 97.5 in; revolutions per mile = 63,360 ÷ 97.5 = 650.
  4. 255/50R20: sidewall = 255 mm × 50% = 127.5 mm = 5.02 in.
  5. Diameter = rim + 2 × sidewall = 20 + 2 × 5.02 = 30.04 in (763 mm).
  6. Circumference = π × 30.04 = 94.4 in; revolutions per mile = 63,360 ÷ 94.4 = 671.
  7. Speedometer. True speed = what it shows × new diameter ÷ old = 60 × 30.04 ÷ 31.04 = 58.1 mph.
  8. Ground clearance. Half the change in diameter: (30.04 − 31.04) ÷ 2 = −0.50 in.

Where the 255/55R20 numbers come from

The section width is 255 mm (10.04 inches), and each sidewall is 55% of it: 140.3 mm, or 5.52 inches. Two sidewalls and the 20-inch rim make 31.04 inches (789 mm).

One turn covers π × 31.04 = 97.5 inches, so the tire turns 650 times a mile and 404 times a kilometer.

What tire makers publish for 255/55R20

Inflated on the maker’s measuring rim. The percentage is the printed diameter against the 31.04 inches worked out above.

TireSize as printedDiameterWidthRevs per mile
Toyo Open Country A/T III255/55R2031 in (−0.1%)10.4 in670

Other 20-inch sizes against 255/55R20

Each size’s height against 255/55R20, and what a speedometer set for 255/55R20 shows at 60 mph with that size on.

SizeDiameterWidthHeight changeAt 60 mph shown
245/50R2029.6 in9.6 in−4.5%57.3 mph
255/50R20 compared30.0 in10.0 in−3.2%58.1 mph
275/55R2031.9 in10.8 in+2.8%61.7 mph
275/60R2033.0 in10.8 in+6.3%63.8 mph
275/65R2034.1 in10.8 in+9.8%65.9 mph
285/55R2032.3 in11.2 in+4.2%62.5 mph
285/60R2033.5 in11.2 in+7.8%64.7 mph
33x12.50R2033.0 in12.5 in+6.3%63.8 mph
35x12.50R2035.0 in12.5 in+12.7%67.6 mph

255/55R20 compared

Every size: the tire size calculator. With the wheels off, the nuts go back on at your vehicle’s lug nut torque.

Questions

What is 255/55R20 in inches?

About 31.0x10.0R20: 31.0 inches tall and 10.0 inches wide on a 20-inch wheel, with 5.5-inch sidewalls. Exactly, 20 + 2 × 255 mm × 0.55 ÷ 25.4 = 31.04 inches.

How many revolutions per mile does a 255/55R20 make?

650 by its diameter: 63,360 inches in a mile ÷ 97.5 inches a turn. Tire makers’ published figures are measured under load, when the tire rolls on a slightly smaller radius, and run 2.8% to 3.3% higher in Toyo’s tables.

Is LT255/55R20 the same size as P255/55R20?

In dimensions, yes: the numbers mean the same in every metric size. The letters mark what the tire is built for, and an LT tire’s load is rated differently, so check the load index and pressure your vehicle needs before swapping one for the other.

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.