Tire Size Box

315/45R21 vs 315/50R21

315/50R21 compared with 315/45R21

+3.9% diameter, 0.0" width

315/50R21 is 33.40" tall vs 32.16". More than the 3% rule of thumb, so check carefully before switching.

32.2"21" rim5.6"12.4" wide
315/45R21 (solid) compared with 315/50R21 (dashed outline), drawn to scale

With 315/50R21 tires, when your speedometer says 60 mph you are really going

62.3 mph

315/50R21 is +1.24 inches (+3.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/45R21315/50R21Difference%
Overall diameter32.16"33.40"+1.24"+3.9%
Section width12.40"12.40"0.00"0.0%
Sidewall height5.58"6.20"+0.62"+11.1%
Circumference101.04"104.93"+3.90"+3.9%
Revolutions per mile627.1603.8−23.3−3.7%

Speedometer difference

If your vehicle came with 315/45R21 and you fit 315/50R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.2 mph+1.2 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, 315/45R21 or 315/50R21?
315/50R21 is taller by 1.24 inches (33.40" vs 32.16"). 315/45R21 is the same width.
Can I replace 315/45R21 with 315/50R21?
It is not recommended without checking carefully. The diameter difference is +3.9%, 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 315/50R21 change my speedometer?
When your speedometer reads 60 mph with 315/50R21 fitted (calibrated for 315/45R21), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.9%.
Will 315/50R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.62 inches of ground clearance at the axle.

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