Tire Size Box

235/40R20 vs 235/50R20

235/50R20 compared with 235/40R20

+6.8% diameter, 0.0" width

235/50R20 is 29.25" tall vs 27.40". More than the 3% rule of thumb, so check carefully before switching.

27.4"20" rim3.7"9.3" wide
235/40R20 (solid) compared with 235/50R20 (dashed outline), drawn to scale

With 235/50R20 tires, when your speedometer says 60 mph you are really going

64.1 mph

235/50R20 is +1.85 inches (+6.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/40R20235/50R20Difference%
Overall diameter27.40"29.25"+1.85"+6.8%
Section width9.25"9.25"0.00"0.0%
Sidewall height3.70"4.63"+0.93"+25.0%
Circumference86.08"91.90"+5.81"+6.8%
Revolutions per mile736.0689.5−46.6−6.3%

Speedometer difference

If your vehicle came with 235/40R20 and you fit 235/50R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.4 mph+1.4 mph
30 mph32.0 mph+2.0 mph
40 mph42.7 mph+2.7 mph
50 mph53.4 mph+3.4 mph
60 mph64.1 mph+4.1 mph
65 mph69.4 mph+4.4 mph
70 mph74.7 mph+4.7 mph
75 mph80.1 mph+5.1 mph
80 mph85.4 mph+5.4 mph

Each size on its own

Questions

Which is bigger, 235/40R20 or 235/50R20?
235/50R20 is taller by 1.85 inches (29.25" vs 27.40"). 235/40R20 is the same width.
Can I replace 235/40R20 with 235/50R20?
It is not recommended without checking carefully. The diameter difference is +6.8%, 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 235/50R20 change my speedometer?
When your speedometer reads 60 mph with 235/50R20 fitted (calibrated for 235/40R20), your real speed is 64.1 mph. Your odometer will under-count miles by about 6.8%.
Will 235/50R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.93 inches of ground clearance at the axle.

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