Tire Size Box

325/45R18 vs 325/55R18

325/55R18 compared with 325/45R18

+8.7% diameter, 0.0" width

325/55R18 is 32.07" tall vs 29.52". More than the 3% rule of thumb, so check carefully before switching.

29.5"18" rim5.8"12.8" wide
325/45R18 (solid) compared with 325/55R18 (dashed outline), drawn to scale

With 325/55R18 tires, when your speedometer says 60 mph you are really going

65.2 mph

325/55R18 is +2.56 inches (+8.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/45R18325/55R18Difference%
Overall diameter29.52"32.07"+2.56"+8.7%
Section width12.80"12.80"0.00"0.0%
Sidewall height5.76"7.04"+1.28"+22.2%
Circumference92.73"100.77"+8.04"+8.7%
Revolutions per mile683.3628.8−54.5−8.0%

Speedometer difference

If your vehicle came with 325/45R18 and you fit 325/55R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.6 mph+2.6 mph
40 mph43.5 mph+3.5 mph
50 mph54.3 mph+4.3 mph
60 mph65.2 mph+5.2 mph
65 mph70.6 mph+5.6 mph
70 mph76.1 mph+6.1 mph
75 mph81.5 mph+6.5 mph
80 mph86.9 mph+6.9 mph

Each size on its own

Questions

Which is bigger, 325/45R18 or 325/55R18?
325/55R18 is taller by 2.56 inches (32.07" vs 29.52"). 325/45R18 is the same width.
Can I replace 325/45R18 with 325/55R18?
It is not recommended without checking carefully. The diameter difference is +8.7%, 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 325/55R18 change my speedometer?
When your speedometer reads 60 mph with 325/55R18 fitted (calibrated for 325/45R18), your real speed is 65.2 mph. Your odometer will under-count miles by about 8.7%.
Will 325/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.28 inches of ground clearance at the axle.

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