Tire Size Box

305/50R21 vs 305/55R21

305/55R21 compared with 305/50R21

+3.6% diameter, 0.0" width

305/55R21 is 34.21" tall vs 33.01". More than the 3% rule of thumb, so check carefully before switching.

33.0"21" rim6.0"12.0" wide
305/50R21 (solid) compared with 305/55R21 (dashed outline), drawn to scale

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

62.2 mph

305/55R21 is +1.20 inches (+3.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/50R21305/55R21Difference%
Overall diameter33.01"34.21"+1.20"+3.6%
Section width12.01"12.01"0.00"0.0%
Sidewall height6.00"6.60"+0.60"+10.0%
Circumference103.70"107.47"+3.77"+3.6%
Revolutions per mile611.0589.6−21.4−3.5%

Speedometer difference

If your vehicle came with 305/50R21 and you fit 305/55R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.8 mph+1.8 mph
60 mph62.2 mph+2.2 mph
65 mph67.4 mph+2.4 mph
70 mph72.5 mph+2.5 mph
75 mph77.7 mph+2.7 mph
80 mph82.9 mph+2.9 mph

Each size on its own

Questions

Which is bigger, 305/50R21 or 305/55R21?
305/55R21 is taller by 1.20 inches (34.21" vs 33.01"). 305/50R21 is the same width.
Can I replace 305/50R21 with 305/55R21?
It is not recommended without checking carefully. The diameter difference is +3.6%, 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 305/55R21 change my speedometer?
When your speedometer reads 60 mph with 305/55R21 fitted (calibrated for 305/50R21), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.6%.
Will 305/55R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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