Tire Size Box

325/45R21 vs 325/55R21

325/55R21 compared with 325/45R21

+7.9% diameter, 0.0" width

325/55R21 is 35.07" tall vs 32.52". More than the 3% rule of thumb, so check carefully before switching.

32.5"21" rim5.8"12.8" wide
325/45R21 (solid) compared with 325/55R21 (dashed outline), drawn to scale

With 325/55R21 tires, when your speedometer says 60 mph you are really going

64.7 mph

325/55R21 is +2.56 inches (+7.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/45R21325/55R21Difference%
Overall diameter32.52"35.07"+2.56"+7.9%
Section width12.80"12.80"0.00"0.0%
Sidewall height5.76"7.04"+1.28"+22.2%
Circumference102.15"110.19"+8.04"+7.9%
Revolutions per mile620.3575.0−45.3−7.3%

Speedometer difference

If your vehicle came with 325/45R21 and you fit 325/55R21, 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 mph43.1 mph+3.1 mph
50 mph53.9 mph+3.9 mph
60 mph64.7 mph+4.7 mph
65 mph70.1 mph+5.1 mph
70 mph75.5 mph+5.5 mph
75 mph80.9 mph+5.9 mph
80 mph86.3 mph+6.3 mph

Each size on its own

Questions

Which is bigger, 325/45R21 or 325/55R21?
325/55R21 is taller by 2.56 inches (35.07" vs 32.52"). 325/45R21 is the same width.
Can I replace 325/45R21 with 325/55R21?
It is not recommended without checking carefully. The diameter difference is +7.9%, 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 325/55R21 change my speedometer?
When your speedometer reads 60 mph with 325/55R21 fitted (calibrated for 325/45R21), your real speed is 64.7 mph. Your odometer will under-count miles by about 7.9%.
Will 325/55R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.28 inches of ground clearance at the axle.

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