Tire Size Box

285/55R21 vs 305/55R21

305/55R21 compared with 285/55R21

+2.6% diameter, +0.8" width

305/55R21 is 34.21" tall vs 33.34". Within the 3% rule, so usually interchangeable.

33.3"21" rim6.2"11.2" wide
285/55R21 (solid) compared with 305/55R21 (dashed outline), drawn to scale

With 305/55R21 tires, when your speedometer says 60 mph you are really going

61.6 mph

305/55R21 is +0.87 inches (+2.6%) in overall diameter. That is within the 3% most shops and makers allow.

285/55R21305/55R21Difference%
Overall diameter33.34"34.21"+0.87"+2.6%
Section width11.22"12.01"+0.79"+7.0%
Sidewall height6.17"6.60"+0.43"+7.0%
Circumference104.75"107.47"+2.72"+2.6%
Revolutions per mile604.9589.6−15.3−2.5%

Speedometer difference

If your vehicle came with 285/55R21 and you fit 305/55R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.5 mph+0.5 mph
30 mph30.8 mph+0.8 mph
40 mph41.0 mph+1.0 mph
50 mph51.3 mph+1.3 mph
60 mph61.6 mph+1.6 mph
65 mph66.7 mph+1.7 mph
70 mph71.8 mph+1.8 mph
75 mph76.9 mph+1.9 mph
80 mph82.1 mph+2.1 mph

Each size on its own

Questions

Which is bigger, 285/55R21 or 305/55R21?
305/55R21 is taller by 0.87 inches (34.21" vs 33.34"). 305/55R21 is wider by 0.79 inches.
Can I replace 285/55R21 with 305/55R21?
Usually, yes. The diameter difference is +2.6%, 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 305/55R21 change my speedometer?
When your speedometer reads 60 mph with 305/55R21 fitted (calibrated for 285/55R21), your real speed is 61.6 mph. Your odometer will under-count miles by about 2.6%.
Will 305/55R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.43 inches of ground clearance at the axle.

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