Tire Size Box

315/50R21 vs 315/55R21

315/55R21 compared with 315/50R21

+3.7% diameter, 0.0" width

315/55R21 is 34.64" tall vs 33.40". More than the 3% rule of thumb, so check carefully before switching.

33.4"21" rim6.2"12.4" wide
315/50R21 (solid) compared with 315/55R21 (dashed outline), drawn to scale

With 315/55R21 tires, when your speedometer says 60 mph you are really going

62.2 mph

315/55R21 is +1.24 inches (+3.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/50R21315/55R21Difference%
Overall diameter33.40"34.64"+1.24"+3.7%
Section width12.40"12.40"0.00"0.0%
Sidewall height6.20"6.82"+0.62"+10.0%
Circumference104.93"108.83"+3.90"+3.7%
Revolutions per mile603.8582.2−21.6−3.6%

Speedometer difference

If your vehicle came with 315/50R21 and you fit 315/55R21, 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 mph51.9 mph+1.9 mph
60 mph62.2 mph+2.2 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, 315/50R21 or 315/55R21?
315/55R21 is taller by 1.24 inches (34.64" vs 33.40"). 315/50R21 is the same width.
Can I replace 315/50R21 with 315/55R21?
It is not recommended without checking carefully. The diameter difference is +3.7%, 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 315/55R21 change my speedometer?
When your speedometer reads 60 mph with 315/55R21 fitted (calibrated for 315/50R21), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.7%.
Will 315/55R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.62 inches of ground clearance at the axle.

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