Tire Size Box

255/45R18 vs 255/55R18

255/55R18 compared with 255/45R18

+7.4% diameter, 0.0" width

255/55R18 is 29.04" tall vs 27.04". More than the 3% rule of thumb, so check carefully before switching.

27.0"18" rim4.5"10.0" wide
255/45R18 (solid) compared with 255/55R18 (dashed outline), drawn to scale

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

64.5 mph

255/55R18 is +2.01 inches (+7.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/45R18255/55R18Difference%
Overall diameter27.04"29.04"+2.01"+7.4%
Section width10.04"10.04"0.00"0.0%
Sidewall height4.52"5.52"+1.00"+22.2%
Circumference84.93"91.24"+6.31"+7.4%
Revolutions per mile746.0694.4−51.6−6.9%

Speedometer difference

If your vehicle came with 255/45R18 and you fit 255/55R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.5 mph+1.5 mph
30 mph32.2 mph+2.2 mph
40 mph43.0 mph+3.0 mph
50 mph53.7 mph+3.7 mph
60 mph64.5 mph+4.5 mph
65 mph69.8 mph+4.8 mph
70 mph75.2 mph+5.2 mph
75 mph80.6 mph+5.6 mph
80 mph85.9 mph+5.9 mph

Each size on its own

Questions

Which is bigger, 255/45R18 or 255/55R18?
255/55R18 is taller by 2.01 inches (29.04" vs 27.04"). 255/45R18 is the same width.
Can I replace 255/45R18 with 255/55R18?
It is not recommended without checking carefully. The diameter difference is +7.4%, 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 255/55R18 change my speedometer?
When your speedometer reads 60 mph with 255/55R18 fitted (calibrated for 255/45R18), your real speed is 64.5 mph. Your odometer will under-count miles by about 7.4%.
Will 255/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.00 inches of ground clearance at the axle.

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