Tire Size Box

235/55R18 vs 255/55R18

255/55R18 compared with 235/55R18

+3.1% diameter, +0.8" width

255/55R18 is 29.04" tall vs 28.18". More than the 3% rule of thumb, so check carefully before switching.

28.2"18" rim5.1"9.3" wide
235/55R18 (solid) compared with 255/55R18 (dashed outline), drawn to scale

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

61.8 mph

255/55R18 is +0.87 inches (+3.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/55R18255/55R18Difference%
Overall diameter28.18"29.04"+0.87"+3.1%
Section width9.25"10.04"+0.79"+8.5%
Sidewall height5.09"5.52"+0.43"+8.5%
Circumference88.52"91.24"+2.72"+3.1%
Revolutions per mile715.8694.4−21.3−3.0%

Speedometer difference

If your vehicle came with 235/55R18 and you fit 255/55R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph30.9 mph+0.9 mph
40 mph41.2 mph+1.2 mph
50 mph51.5 mph+1.5 mph
60 mph61.8 mph+1.8 mph
65 mph67.0 mph+2.0 mph
70 mph72.2 mph+2.2 mph
75 mph77.3 mph+2.3 mph
80 mph82.5 mph+2.5 mph

Each size on its own

Questions

Which is bigger, 235/55R18 or 255/55R18?
255/55R18 is taller by 0.87 inches (29.04" vs 28.18"). 255/55R18 is wider by 0.79 inches.
Can I replace 235/55R18 with 255/55R18?
It is not recommended without checking carefully. The diameter difference is +3.1%, 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 235/55R18), your real speed is 61.8 mph. Your odometer will under-count miles by about 3.1%.
Will 255/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.43 inches of ground clearance at the axle.

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