Tire Size Box

225/55R18 vs 245/55R18

245/55R18 compared with 225/55R18

+3.1% diameter, +0.8" width

245/55R18 is 28.61" tall vs 27.74". More than the 3% rule of thumb, so check carefully before switching.

27.7"18" rim4.9"8.9" wide
225/55R18 (solid) compared with 245/55R18 (dashed outline), drawn to scale

With 245/55R18 tires, when your speedometer says 60 mph you are really going

61.9 mph

245/55R18 is +0.87 inches (+3.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

225/55R18245/55R18Difference%
Overall diameter27.74"28.61"+0.87"+3.1%
Section width8.86"9.65"+0.79"+8.9%
Sidewall height4.87"5.31"+0.43"+8.9%
Circumference87.16"89.88"+2.72"+3.1%
Revolutions per mile726.9704.9−22.0−3.0%

Speedometer difference

If your vehicle came with 225/55R18 and you fit 245/55R18, 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 mph51.6 mph+1.6 mph
60 mph61.9 mph+1.9 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, 225/55R18 or 245/55R18?
245/55R18 is taller by 0.87 inches (28.61" vs 27.74"). 245/55R18 is wider by 0.79 inches.
Can I replace 225/55R18 with 245/55R18?
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 245/55R18 change my speedometer?
When your speedometer reads 60 mph with 245/55R18 fitted (calibrated for 225/55R18), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.1%.
Will 245/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.43 inches of ground clearance at the axle.

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