Tire Size Box

325/40R21 vs 325/50R21

325/50R21 compared with 325/40R21

+8.2% diameter, 0.0" width

325/50R21 is 33.80" tall vs 31.24". More than the 3% rule of thumb, so check carefully before switching.

31.2"21" rim5.1"12.8" wide
325/40R21 (solid) compared with 325/50R21 (dashed outline), drawn to scale

With 325/50R21 tires, when your speedometer says 60 mph you are really going

64.9 mph

325/50R21 is +2.56 inches (+8.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/40R21325/50R21Difference%
Overall diameter31.24"33.80"+2.56"+8.2%
Section width12.80"12.80"0.00"0.0%
Sidewall height5.12"6.40"+1.28"+25.0%
Circumference98.13"106.17"+8.04"+8.2%
Revolutions per mile645.7596.8−48.9−7.6%

Speedometer difference

If your vehicle came with 325/40R21 and you fit 325/50R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.5 mph+2.5 mph
40 mph43.3 mph+3.3 mph
50 mph54.1 mph+4.1 mph
60 mph64.9 mph+4.9 mph
65 mph70.3 mph+5.3 mph
70 mph75.7 mph+5.7 mph
75 mph81.1 mph+6.1 mph
80 mph86.6 mph+6.6 mph

Each size on its own

Questions

Which is bigger, 325/40R21 or 325/50R21?
325/50R21 is taller by 2.56 inches (33.80" vs 31.24"). 325/40R21 is the same width.
Can I replace 325/40R21 with 325/50R21?
It is not recommended without checking carefully. The diameter difference is +8.2%, 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 325/50R21 change my speedometer?
When your speedometer reads 60 mph with 325/50R21 fitted (calibrated for 325/40R21), your real speed is 64.9 mph. Your odometer will under-count miles by about 8.2%.
Will 325/50R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.28 inches of ground clearance at the axle.

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