Tire Size Box

315/40R21 vs 315/50R21

315/50R21 compared with 315/40R21

+8.0% diameter, 0.0" width

315/50R21 is 33.40" tall vs 30.92". More than the 3% rule of thumb, so check carefully before switching.

30.9"21" rim5.0"12.4" wide
315/40R21 (solid) compared with 315/50R21 (dashed outline), drawn to scale

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

64.8 mph

315/50R21 is +2.48 inches (+8.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/40R21315/50R21Difference%
Overall diameter30.92"33.40"+2.48"+8.0%
Section width12.40"12.40"0.00"0.0%
Sidewall height4.96"6.20"+1.24"+25.0%
Circumference97.14"104.93"+7.79"+8.0%
Revolutions per mile652.2603.8−48.4−7.4%

Speedometer difference

If your vehicle came with 315/40R21 and you fit 315/50R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.4 mph+2.4 mph
40 mph43.2 mph+3.2 mph
50 mph54.0 mph+4.0 mph
60 mph64.8 mph+4.8 mph
65 mph70.2 mph+5.2 mph
70 mph75.6 mph+5.6 mph
75 mph81.0 mph+6.0 mph
80 mph86.4 mph+6.4 mph

Each size on its own

Questions

Which is bigger, 315/40R21 or 315/50R21?
315/50R21 is taller by 2.48 inches (33.40" vs 30.92"). 315/40R21 is the same width.
Can I replace 315/40R21 with 315/50R21?
It is not recommended without checking carefully. The diameter difference is +8.0%, 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 315/50R21 change my speedometer?
When your speedometer reads 60 mph with 315/50R21 fitted (calibrated for 315/40R21), your real speed is 64.8 mph. Your odometer will under-count miles by about 8.0%.
Will 315/50R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.24 inches of ground clearance at the axle.

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