Tire Size Box

255/45R18 vs 255/50R18

255/50R18 compared with 255/45R18

+3.7% diameter, 0.0" width

255/50R18 is 28.04" 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 255/50R18 (dashed outline), drawn to scale

With 255/50R18 tires, when your speedometer says 60 mph you are really going

62.2 mph

255/50R18 is +1.00 inches (+3.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/45R18255/50R18Difference%
Overall diameter27.04"28.04"+1.00"+3.7%
Section width10.04"10.04"0.00"0.0%
Sidewall height4.52"5.02"+0.50"+11.1%
Circumference84.93"88.09"+3.15"+3.7%
Revolutions per mile746.0719.3−26.7−3.6%

Speedometer difference

If your vehicle came with 255/45R18 and you fit 255/50R18, 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 mph41.5 mph+1.5 mph
50 mph51.9 mph+1.9 mph
60 mph62.2 mph+2.2 mph
65 mph67.4 mph+2.4 mph
70 mph72.6 mph+2.6 mph
75 mph77.8 mph+2.8 mph
80 mph83.0 mph+3.0 mph

Each size on its own

Questions

Which is bigger, 255/45R18 or 255/50R18?
255/50R18 is taller by 1.00 inches (28.04" vs 27.04"). 255/45R18 is the same width.
Can I replace 255/45R18 with 255/50R18?
It is not recommended without checking carefully. The diameter difference is +3.7%, 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 255/50R18 change my speedometer?
When your speedometer reads 60 mph with 255/50R18 fitted (calibrated for 255/45R18), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.7%.
Will 255/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

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