Tire Size Box

315/40R20 vs 315/50R20

315/50R20 compared with 315/40R20

+8.3% diameter, 0.0" width

315/50R20 is 32.40" tall vs 29.92". More than the 3% rule of thumb, so check carefully before switching.

29.9"20" rim5.0"12.4" wide
315/40R20 (solid) compared with 315/50R20 (dashed outline), drawn to scale

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

65.0 mph

315/50R20 is +2.48 inches (+8.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/40R20315/50R20Difference%
Overall diameter29.92"32.40"+2.48"+8.3%
Section width12.40"12.40"0.00"0.0%
Sidewall height4.96"6.20"+1.24"+25.0%
Circumference94.00"101.79"+7.79"+8.3%
Revolutions per mile674.0622.4−51.6−7.7%

Speedometer difference

If your vehicle came with 315/40R20 and you fit 315/50R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.5 mph+2.5 mph
40 mph43.3 mph+3.3 mph
50 mph54.1 mph+4.1 mph
60 mph65.0 mph+5.0 mph
65 mph70.4 mph+5.4 mph
70 mph75.8 mph+5.8 mph
75 mph81.2 mph+6.2 mph
80 mph86.6 mph+6.6 mph

Each size on its own

Questions

Which is bigger, 315/40R20 or 315/50R20?
315/50R20 is taller by 2.48 inches (32.40" vs 29.92"). 315/40R20 is the same width.
Can I replace 315/40R20 with 315/50R20?
It is not recommended without checking carefully. The diameter difference is +8.3%, 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 315/40R20), your real speed is 65.0 mph. Your odometer will under-count miles by about 8.3%.
Will 315/50R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.24 inches of ground clearance at the axle.

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