Tire Size Box

285/55R20 vs 285/50R21

285/50R21 compared with 285/55R20

−0.4% diameter, 0.0" width

285/50R21 is 32.22" tall vs 32.34". Within the 3% rule, so usually interchangeable.

32.3"20" rim6.2"11.2" wide
285/55R20 (solid) compared with 285/50R21 (dashed outline), drawn to scale

With 285/50R21 tires, when your speedometer says 60 mph you are really going

59.8 mph

285/50R21 is −0.12 inches (−0.4%) in overall diameter. That is within the 3% most shops and makers allow.

285/55R20285/50R21Difference%
Overall diameter32.34"32.22"−0.12"−0.4%
Section width11.22"11.22"0.00"0.0%
Sidewall height6.17"5.61"−0.56"−9.1%
Circumference101.61"101.22"−0.38"−0.4%
Revolutions per mile623.6625.9+2.4+0.4%

Speedometer difference

If your vehicle came with 285/55R20 and you fit 285/50R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph19.9 mph−0.1 mph
30 mph29.9 mph−0.1 mph
40 mph39.8 mph−0.2 mph
50 mph49.8 mph−0.2 mph
60 mph59.8 mph−0.2 mph
65 mph64.8 mph−0.2 mph
70 mph69.7 mph−0.3 mph
75 mph74.7 mph−0.3 mph
80 mph79.7 mph−0.3 mph

Each size on its own

Questions

Which is bigger, 285/55R20 or 285/50R21?
285/55R20 is taller by 0.12 inches (32.34" vs 32.22"). 285/55R20 is the same width.
Can I replace 285/55R20 with 285/50R21?
Usually, yes. The diameter difference is −0.4%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 285/50R21 change my speedometer?
When your speedometer reads 60 mph with 285/50R21 fitted (calibrated for 285/55R20), your real speed is 59.8 mph. Your odometer will over-count miles by about 0.4%.
Will 285/50R21 lift my vehicle?
Ride height changes by half the diameter difference: −0.06 inches of ground clearance at the axle.

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