Tire Size Box

325/45R20 vs 325/55R20

325/55R20 compared with 325/45R20

+8.1% diameter, 0.0" width

325/55R20 is 34.07" tall vs 31.52". More than the 3% rule of thumb, so check carefully before switching.

31.5"20" rim5.8"12.8" wide
325/45R20 (solid) compared with 325/55R20 (dashed outline), drawn to scale

With 325/55R20 tires, when your speedometer says 60 mph you are really going

64.9 mph

325/55R20 is +2.56 inches (+8.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/45R20325/55R20Difference%
Overall diameter31.52"34.07"+2.56"+8.1%
Section width12.80"12.80"0.00"0.0%
Sidewall height5.76"7.04"+1.28"+22.2%
Circumference99.01"107.05"+8.04"+8.1%
Revolutions per mile639.9591.9−48.1−7.5%

Speedometer difference

If your vehicle came with 325/45R20 and you fit 325/55R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.4 mph+2.4 mph
40 mph43.2 mph+3.2 mph
50 mph54.1 mph+4.1 mph
60 mph64.9 mph+4.9 mph
65 mph70.3 mph+5.3 mph
70 mph75.7 mph+5.7 mph
75 mph81.1 mph+6.1 mph
80 mph86.5 mph+6.5 mph

Each size on its own

Questions

Which is bigger, 325/45R20 or 325/55R20?
325/55R20 is taller by 2.56 inches (34.07" vs 31.52"). 325/45R20 is the same width.
Can I replace 325/45R20 with 325/55R20?
It is not recommended without checking carefully. The diameter difference is +8.1%, 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 325/55R20 change my speedometer?
When your speedometer reads 60 mph with 325/55R20 fitted (calibrated for 325/45R20), your real speed is 64.9 mph. Your odometer will under-count miles by about 8.1%.
Will 325/55R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.28 inches of ground clearance at the axle.

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