Tire Size Box

315/50R20 vs 315/55R20

315/55R20 compared with 315/50R20

+3.8% diameter, 0.0" width

315/55R20 is 33.64" tall vs 32.40". More than the 3% rule of thumb, so check carefully before switching.

32.4"20" rim6.2"12.4" wide
315/50R20 (solid) compared with 315/55R20 (dashed outline), drawn to scale

With 315/55R20 tires, when your speedometer says 60 mph you are really going

62.3 mph

315/55R20 is +1.24 inches (+3.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/50R20315/55R20Difference%
Overall diameter32.40"33.64"+1.24"+3.8%
Section width12.40"12.40"0.00"0.0%
Sidewall height6.20"6.82"+0.62"+10.0%
Circumference101.79"105.69"+3.90"+3.8%
Revolutions per mile622.4599.5−22.9−3.7%

Speedometer difference

If your vehicle came with 315/50R20 and you fit 315/55R20, 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 mph67.5 mph+2.5 mph
70 mph72.7 mph+2.7 mph
75 mph77.9 mph+2.9 mph
80 mph83.1 mph+3.1 mph

Each size on its own

Questions

Which is bigger, 315/50R20 or 315/55R20?
315/55R20 is taller by 1.24 inches (33.64" vs 32.40"). 315/50R20 is the same width.
Can I replace 315/50R20 with 315/55R20?
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 315/55R20 change my speedometer?
When your speedometer reads 60 mph with 315/55R20 fitted (calibrated for 315/50R20), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.8%.
Will 315/55R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.62 inches of ground clearance at the axle.

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