Tire Size Box

355/40R20 vs 355/50R20

355/50R20 compared with 355/40R20

+9.0% diameter, 0.0" width

355/50R20 is 33.98" tall vs 31.18". More than the 3% rule of thumb, so check carefully before switching.

31.2"20" rim5.6"14.0" wide
355/40R20 (solid) compared with 355/50R20 (dashed outline), drawn to scale

With 355/50R20 tires, when your speedometer says 60 mph you are really going

65.4 mph

355/50R20 is +2.80 inches (+9.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/40R20355/50R20Difference%
Overall diameter31.18"33.98"+2.80"+9.0%
Section width13.98"13.98"0.00"0.0%
Sidewall height5.59"6.99"+1.40"+25.0%
Circumference97.96"106.74"+8.78"+9.0%
Revolutions per mile646.8593.6−53.2−8.2%

Speedometer difference

If your vehicle came with 355/40R20 and you fit 355/50R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.8 mph+1.8 mph
30 mph32.7 mph+2.7 mph
40 mph43.6 mph+3.6 mph
50 mph54.5 mph+4.5 mph
60 mph65.4 mph+5.4 mph
65 mph70.8 mph+5.8 mph
70 mph76.3 mph+6.3 mph
75 mph81.7 mph+6.7 mph
80 mph87.2 mph+7.2 mph

Each size on its own

Questions

Which is bigger, 355/40R20 or 355/50R20?
355/50R20 is taller by 2.80 inches (33.98" vs 31.18"). 355/40R20 is the same width.
Can I replace 355/40R20 with 355/50R20?
It is not recommended without checking carefully. The diameter difference is +9.0%, 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 355/50R20 change my speedometer?
When your speedometer reads 60 mph with 355/50R20 fitted (calibrated for 355/40R20), your real speed is 65.4 mph. Your odometer will under-count miles by about 9.0%.
Will 355/50R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.40 inches of ground clearance at the axle.

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