Tire Size Box

255/40R20 vs 255/50R20

255/50R20 compared with 255/40R20

+7.2% diameter, 0.0" width

255/50R20 is 30.04" tall vs 28.03". More than the 3% rule of thumb, so check carefully before switching.

28.0"20" rim4.0"10.0" wide
255/40R20 (solid) compared with 255/50R20 (dashed outline), drawn to scale

With 255/50R20 tires, when your speedometer says 60 mph you are really going

64.3 mph

255/50R20 is +2.01 inches (+7.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/40R20255/50R20Difference%
Overall diameter28.03"30.04"+2.01"+7.2%
Section width10.04"10.04"0.00"0.0%
Sidewall height4.02"5.02"+1.00"+25.0%
Circumference88.06"94.37"+6.31"+7.2%
Revolutions per mile719.5671.4−48.1−6.7%

Speedometer difference

If your vehicle came with 255/40R20 and you fit 255/50R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.4 mph+1.4 mph
30 mph32.1 mph+2.1 mph
40 mph42.9 mph+2.9 mph
50 mph53.6 mph+3.6 mph
60 mph64.3 mph+4.3 mph
65 mph69.7 mph+4.7 mph
70 mph75.0 mph+5.0 mph
75 mph80.4 mph+5.4 mph
80 mph85.7 mph+5.7 mph

Each size on its own

Questions

Which is bigger, 255/40R20 or 255/50R20?
255/50R20 is taller by 2.01 inches (30.04" vs 28.03"). 255/40R20 is the same width.
Can I replace 255/40R20 with 255/50R20?
It is not recommended without checking carefully. The diameter difference is +7.2%, 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 255/50R20 change my speedometer?
When your speedometer reads 60 mph with 255/50R20 fitted (calibrated for 255/40R20), your real speed is 64.3 mph. Your odometer will under-count miles by about 7.2%.
Will 255/50R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.00 inches of ground clearance at the axle.

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