Tire Size Box

255/50R18 vs 255/55R18

255/55R18 compared with 255/50R18

+3.6% diameter, 0.0" width

255/55R18 is 29.04" tall vs 28.04". More than the 3% rule of thumb, so check carefully before switching.

28.0"18" rim5.0"10.0" wide
255/50R18 (solid) compared with 255/55R18 (dashed outline), drawn to scale

With 255/55R18 tires, when your speedometer says 60 mph you are really going

62.1 mph

255/55R18 is +1.00 inches (+3.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/50R18255/55R18Difference%
Overall diameter28.04"29.04"+1.00"+3.6%
Section width10.04"10.04"0.00"0.0%
Sidewall height5.02"5.52"+0.50"+10.0%
Circumference88.09"91.24"+3.15"+3.6%
Revolutions per mile719.3694.4−24.9−3.5%

Speedometer difference

If your vehicle came with 255/50R18 and you fit 255/55R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.4 mph+1.4 mph
50 mph51.8 mph+1.8 mph
60 mph62.1 mph+2.1 mph
65 mph67.3 mph+2.3 mph
70 mph72.5 mph+2.5 mph
75 mph77.7 mph+2.7 mph
80 mph82.9 mph+2.9 mph

Each size on its own

Questions

Which is bigger, 255/50R18 or 255/55R18?
255/55R18 is taller by 1.00 inches (29.04" vs 28.04"). 255/50R18 is the same width.
Can I replace 255/50R18 with 255/55R18?
It is not recommended without checking carefully. The diameter difference is +3.6%, 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 255/55R18 change my speedometer?
When your speedometer reads 60 mph with 255/55R18 fitted (calibrated for 255/50R18), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.6%.
Will 255/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

Other comparisons