Tire Size Box

325/50R21 vs 325/55R21

325/55R21 compared with 325/50R21

+3.8% diameter, 0.0" width

325/55R21 is 35.07" tall vs 33.80". More than the 3% rule of thumb, so check carefully before switching.

33.8"21" rim6.4"12.8" wide
325/50R21 (solid) compared with 325/55R21 (dashed outline), drawn to scale

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

62.3 mph

325/55R21 is +1.28 inches (+3.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/50R21325/55R21Difference%
Overall diameter33.80"35.07"+1.28"+3.8%
Section width12.80"12.80"0.00"0.0%
Sidewall height6.40"7.04"+0.64"+10.0%
Circumference106.17"110.19"+4.02"+3.8%
Revolutions per mile596.8575.0−21.8−3.6%

Speedometer difference

If your vehicle came with 325/50R21 and you fit 325/55R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.9 mph+1.9 mph
60 mph62.3 mph+2.3 mph
65 mph67.5 mph+2.5 mph
70 mph72.7 mph+2.7 mph
75 mph77.8 mph+2.8 mph
80 mph83.0 mph+3.0 mph

Each size on its own

Questions

Which is bigger, 325/50R21 or 325/55R21?
325/55R21 is taller by 1.28 inches (35.07" vs 33.80"). 325/50R21 is the same width.
Can I replace 325/50R21 with 325/55R21?
It is not recommended without checking carefully. The diameter difference is +3.8%, 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 325/50R21), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.8%.
Will 325/55R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.64 inches of ground clearance at the axle.

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