Tire Size Box

255/50R21 vs 255/55R21

255/55R21 compared with 255/50R21

+3.2% diameter, 0.0" width

255/55R21 is 32.04" tall vs 31.04". More than the 3% rule of thumb, so check carefully before switching.

31.0"21" rim5.0"10.0" wide
255/50R21 (solid) compared with 255/55R21 (dashed outline), drawn to scale

With 255/55R21 tires, when your speedometer says 60 mph you are really going

61.9 mph

255/55R21 is +1.00 inches (+3.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/50R21255/55R21Difference%
Overall diameter31.04"32.04"+1.00"+3.2%
Section width10.04"10.04"0.00"0.0%
Sidewall height5.02"5.52"+0.50"+10.0%
Circumference97.51"100.67"+3.15"+3.2%
Revolutions per mile649.8629.4−20.4−3.1%

Speedometer difference

If your vehicle came with 255/50R21 and you fit 255/55R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.6 mph+1.6 mph
60 mph61.9 mph+1.9 mph
65 mph67.1 mph+2.1 mph
70 mph72.3 mph+2.3 mph
75 mph77.4 mph+2.4 mph
80 mph82.6 mph+2.6 mph

Each size on its own

Questions

Which is bigger, 255/50R21 or 255/55R21?
255/55R21 is taller by 1.00 inches (32.04" vs 31.04"). 255/50R21 is the same width.
Can I replace 255/50R21 with 255/55R21?
It is not recommended without checking carefully. The diameter difference is +3.2%, 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 255/55R21 change my speedometer?
When your speedometer reads 60 mph with 255/55R21 fitted (calibrated for 255/50R21), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.2%.
Will 255/55R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

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