Tire Size Box

255/50R20 vs 255/55R20

255/55R20 compared with 255/50R20

+3.3% diameter, 0.0" width

255/55R20 is 31.04" tall vs 30.04". More than the 3% rule of thumb, so check carefully before switching.

30.0"20" rim5.0"10.0" wide
255/50R20 (solid) compared with 255/55R20 (dashed outline), drawn to scale

With 255/55R20 tires, when your speedometer says 60 mph you are really going

62.0 mph

255/55R20 is +1.00 inches (+3.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/50R20255/55R20Difference%
Overall diameter30.04"31.04"+1.00"+3.3%
Section width10.04"10.04"0.00"0.0%
Sidewall height5.02"5.52"+0.50"+10.0%
Circumference94.37"97.53"+3.15"+3.3%
Revolutions per mile671.4649.7−21.7−3.2%

Speedometer difference

If your vehicle came with 255/50R20 and you fit 255/55R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.7 mph+1.7 mph
60 mph62.0 mph+2.0 mph
65 mph67.2 mph+2.2 mph
70 mph72.3 mph+2.3 mph
75 mph77.5 mph+2.5 mph
80 mph82.7 mph+2.7 mph

Each size on its own

Questions

Which is bigger, 255/50R20 or 255/55R20?
255/55R20 is taller by 1.00 inches (31.04" vs 30.04"). 255/50R20 is the same width.
Can I replace 255/50R20 with 255/55R20?
It is not recommended without checking carefully. The diameter difference is +3.3%, more than the 3% most shops and makers allow. It will throw off the speedometer and may affect ABS, stability control and clearance.
How much will 255/55R20 change my speedometer?
When your speedometer reads 60 mph with 255/55R20 fitted (calibrated for 255/50R20), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.3%.
Will 255/55R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

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