Tire Size Box

285/40R20 vs 285/50R20

285/50R20 compared with 285/40R20

+7.7% diameter, 0.0" width

285/50R20 is 31.22" tall vs 28.98". More than the 3% rule of thumb, so check carefully before switching.

29.0"20" rim4.5"11.2" wide
285/40R20 (solid) compared with 285/50R20 (dashed outline), drawn to scale

With 285/50R20 tires, when your speedometer says 60 mph you are really going

64.6 mph

285/50R20 is +2.24 inches (+7.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/40R20285/50R20Difference%
Overall diameter28.98"31.22"+2.24"+7.7%
Section width11.22"11.22"0.00"0.0%
Sidewall height4.49"5.61"+1.12"+25.0%
Circumference91.03"98.08"+7.05"+7.7%
Revolutions per mile696.0646.0−50.0−7.2%

Speedometer difference

If your vehicle came with 285/40R20 and you fit 285/50R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.5 mph+1.5 mph
30 mph32.3 mph+2.3 mph
40 mph43.1 mph+3.1 mph
50 mph53.9 mph+3.9 mph
60 mph64.6 mph+4.6 mph
65 mph70.0 mph+5.0 mph
70 mph75.4 mph+5.4 mph
75 mph80.8 mph+5.8 mph
80 mph86.2 mph+6.2 mph

Each size on its own

Questions

Which is bigger, 285/40R20 or 285/50R20?
285/50R20 is taller by 2.24 inches (31.22" vs 28.98"). 285/40R20 is the same width.
Can I replace 285/40R20 with 285/50R20?
It is not recommended without checking carefully. The diameter difference is +7.7%, 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 285/50R20 change my speedometer?
When your speedometer reads 60 mph with 285/50R20 fitted (calibrated for 285/40R20), your real speed is 64.6 mph. Your odometer will under-count miles by about 7.7%.
Will 285/50R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.12 inches of ground clearance at the axle.

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