Tire Size Box

315/45R20 vs 315/50R20

315/50R20 compared with 315/45R20

+4.0% diameter, 0.0" width

315/50R20 is 32.40" tall vs 31.16". More than the 3% rule of thumb, so check carefully before switching.

31.2"20" rim5.6"12.4" wide
315/45R20 (solid) compared with 315/50R20 (dashed outline), drawn to scale

With 315/50R20 tires, when your speedometer says 60 mph you are really going

62.4 mph

315/50R20 is +1.24 inches (+4.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/45R20315/50R20Difference%
Overall diameter31.16"32.40"+1.24"+4.0%
Section width12.40"12.40"0.00"0.0%
Sidewall height5.58"6.20"+0.62"+11.1%
Circumference97.90"101.79"+3.90"+4.0%
Revolutions per mile647.2622.4−24.8−3.8%

Speedometer difference

If your vehicle came with 315/45R20 and you fit 315/50R20, 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 mph41.6 mph+1.6 mph
50 mph52.0 mph+2.0 mph
60 mph62.4 mph+2.4 mph
65 mph67.6 mph+2.6 mph
70 mph72.8 mph+2.8 mph
75 mph78.0 mph+3.0 mph
80 mph83.2 mph+3.2 mph

Each size on its own

Questions

Which is bigger, 315/45R20 or 315/50R20?
315/50R20 is taller by 1.24 inches (32.40" vs 31.16"). 315/45R20 is the same width.
Can I replace 315/45R20 with 315/50R20?
It is not recommended without checking carefully. The diameter difference is +4.0%, 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 315/50R20 change my speedometer?
When your speedometer reads 60 mph with 315/50R20 fitted (calibrated for 315/45R20), your real speed is 62.4 mph. Your odometer will under-count miles by about 4.0%.
Will 315/50R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.62 inches of ground clearance at the axle.

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