Tire Size Box

325/40R20 vs 325/50R20

325/50R20 compared with 325/40R20

+8.5% diameter, 0.0" width

325/50R20 is 32.80" tall vs 30.24". More than the 3% rule of thumb, so check carefully before switching.

30.2"20" rim5.1"12.8" wide
325/40R20 (solid) compared with 325/50R20 (dashed outline), drawn to scale

With 325/50R20 tires, when your speedometer says 60 mph you are really going

65.1 mph

325/50R20 is +2.56 inches (+8.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/40R20325/50R20Difference%
Overall diameter30.24"32.80"+2.56"+8.5%
Section width12.80"12.80"0.00"0.0%
Sidewall height5.12"6.40"+1.28"+25.0%
Circumference94.99"103.03"+8.04"+8.5%
Revolutions per mile667.0615.0−52.0−7.8%

Speedometer difference

If your vehicle came with 325/40R20 and you fit 325/50R20, 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 mph43.4 mph+3.4 mph
50 mph54.2 mph+4.2 mph
60 mph65.1 mph+5.1 mph
65 mph70.5 mph+5.5 mph
70 mph75.9 mph+5.9 mph
75 mph81.3 mph+6.3 mph
80 mph86.8 mph+6.8 mph

Each size on its own

Questions

Which is bigger, 325/40R20 or 325/50R20?
325/50R20 is taller by 2.56 inches (32.80" vs 30.24"). 325/40R20 is the same width.
Can I replace 325/40R20 with 325/50R20?
It is not recommended without checking carefully. The diameter difference is +8.5%, 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 325/50R20 change my speedometer?
When your speedometer reads 60 mph with 325/50R20 fitted (calibrated for 325/40R20), your real speed is 65.1 mph. Your odometer will under-count miles by about 8.5%.
Will 325/50R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.28 inches of ground clearance at the axle.

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