Tire Size Box

235/45R18 vs 235/55R18

235/55R18 compared with 235/45R18

+7.0% diameter, 0.0" width

235/55R18 is 28.18" tall vs 26.33". More than the 3% rule of thumb, so check carefully before switching.

26.3"18" rim4.2"9.3" wide
235/45R18 (solid) compared with 235/55R18 (dashed outline), drawn to scale

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

64.2 mph

235/55R18 is +1.85 inches (+7.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/45R18235/55R18Difference%
Overall diameter26.33"28.18"+1.85"+7.0%
Section width9.25"9.25"0.00"0.0%
Sidewall height4.16"5.09"+0.93"+22.2%
Circumference82.71"88.52"+5.81"+7.0%
Revolutions per mile766.1715.8−50.3−6.6%

Speedometer difference

If your vehicle came with 235/45R18 and you fit 235/55R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.4 mph+1.4 mph
30 mph32.1 mph+2.1 mph
40 mph42.8 mph+2.8 mph
50 mph53.5 mph+3.5 mph
60 mph64.2 mph+4.2 mph
65 mph69.6 mph+4.6 mph
70 mph74.9 mph+4.9 mph
75 mph80.3 mph+5.3 mph
80 mph85.6 mph+5.6 mph

Each size on its own

Questions

Which is bigger, 235/45R18 or 235/55R18?
235/55R18 is taller by 1.85 inches (28.18" vs 26.33"). 235/45R18 is the same width.
Can I replace 235/45R18 with 235/55R18?
It is not recommended without checking carefully. The diameter difference is +7.0%, 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 235/45R18), your real speed is 64.2 mph. Your odometer will under-count miles by about 7.0%.
Will 235/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.93 inches of ground clearance at the axle.

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