Tire Size Box

305/50R20 vs 305/55R20

305/55R20 compared with 305/50R20

+3.8% diameter, 0.0" width

305/55R20 is 33.21" tall vs 32.01". More than the 3% rule of thumb, so check carefully before switching.

32.0"20" rim6.0"12.0" wide
305/50R20 (solid) compared with 305/55R20 (dashed outline), drawn to scale

With 305/55R20 tires, when your speedometer says 60 mph you are really going

62.3 mph

305/55R20 is +1.20 inches (+3.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/50R20305/55R20Difference%
Overall diameter32.01"33.21"+1.20"+3.8%
Section width12.01"12.01"0.00"0.0%
Sidewall height6.00"6.60"+0.60"+10.0%
Circumference100.56"104.33"+3.77"+3.8%
Revolutions per mile630.1607.3−22.8−3.6%

Speedometer difference

If your vehicle came with 305/50R20 and you fit 305/55R20, 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 mph67.4 mph+2.4 mph
70 mph72.6 mph+2.6 mph
75 mph77.8 mph+2.8 mph
80 mph83.0 mph+3.0 mph

Each size on its own

Questions

Which is bigger, 305/50R20 or 305/55R20?
305/55R20 is taller by 1.20 inches (33.21" vs 32.01"). 305/50R20 is the same width.
Can I replace 305/50R20 with 305/55R20?
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 305/55R20 change my speedometer?
When your speedometer reads 60 mph with 305/55R20 fitted (calibrated for 305/50R20), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.8%.
Will 305/55R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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