Tire Size Box

235/50R18 vs 235/55R18

235/55R18 compared with 235/50R18

+3.4% diameter, 0.0" width

235/55R18 is 28.18" tall vs 27.25". More than the 3% rule of thumb, so check carefully before switching.

27.3"18" rim4.6"9.3" wide
235/50R18 (solid) compared with 235/55R18 (dashed outline), drawn to scale

With 235/55R18 tires, when your speedometer says 60 mph you are really going

62.0 mph

235/55R18 is +0.93 inches (+3.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/50R18235/55R18Difference%
Overall diameter27.25"28.18"+0.93"+3.4%
Section width9.25"9.25"0.00"0.0%
Sidewall height4.63"5.09"+0.46"+10.0%
Circumference85.61"88.52"+2.91"+3.4%
Revolutions per mile740.1715.8−24.3−3.3%

Speedometer difference

If your vehicle came with 235/50R18 and you fit 235/55R18, 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 mph41.4 mph+1.4 mph
50 mph51.7 mph+1.7 mph
60 mph62.0 mph+2.0 mph
65 mph67.2 mph+2.2 mph
70 mph72.4 mph+2.4 mph
75 mph77.5 mph+2.5 mph
80 mph82.7 mph+2.7 mph

Each size on its own

Questions

Which is bigger, 235/50R18 or 235/55R18?
235/55R18 is taller by 0.93 inches (28.18" vs 27.25"). 235/50R18 is the same width.
Can I replace 235/50R18 with 235/55R18?
It is not recommended without checking carefully. The diameter difference is +3.4%, 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 235/55R18 change my speedometer?
When your speedometer reads 60 mph with 235/55R18 fitted (calibrated for 235/50R18), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.4%.
Will 235/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

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