Tire Size Box

355/40R21 vs 355/50R21

355/50R21 compared with 355/40R21

+8.7% diameter, 0.0" width

355/50R21 is 34.98" tall vs 32.18". More than the 3% rule of thumb, so check carefully before switching.

32.2"21" rim5.6"14.0" wide
355/40R21 (solid) compared with 355/50R21 (dashed outline), drawn to scale

With 355/50R21 tires, when your speedometer says 60 mph you are really going

65.2 mph

355/50R21 is +2.80 inches (+8.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/40R21355/50R21Difference%
Overall diameter32.18"34.98"+2.80"+8.7%
Section width13.98"13.98"0.00"0.0%
Sidewall height5.59"6.99"+1.40"+25.0%
Circumference101.10"109.88"+8.78"+8.7%
Revolutions per mile626.7576.6−50.1−8.0%

Speedometer difference

If your vehicle came with 355/40R21 and you fit 355/50R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.6 mph+2.6 mph
40 mph43.5 mph+3.5 mph
50 mph54.3 mph+4.3 mph
60 mph65.2 mph+5.2 mph
65 mph70.6 mph+5.6 mph
70 mph76.1 mph+6.1 mph
75 mph81.5 mph+6.5 mph
80 mph86.9 mph+6.9 mph

Each size on its own

Questions

Which is bigger, 355/40R21 or 355/50R21?
355/50R21 is taller by 2.80 inches (34.98" vs 32.18"). 355/40R21 is the same width.
Can I replace 355/40R21 with 355/50R21?
It is not recommended without checking carefully. The diameter difference is +8.7%, 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 355/50R21 change my speedometer?
When your speedometer reads 60 mph with 355/50R21 fitted (calibrated for 355/40R21), your real speed is 65.2 mph. Your odometer will under-count miles by about 8.7%.
Will 355/50R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.40 inches of ground clearance at the axle.

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