Tire Size Box

345/40R20 vs 345/50R20

345/50R20 compared with 345/40R20

+8.8% diameter, 0.0" width

345/50R20 is 33.58" tall vs 30.87". More than the 3% rule of thumb, so check carefully before switching.

30.9"20" rim5.4"13.6" wide
345/40R20 (solid) compared with 345/50R20 (dashed outline), drawn to scale

With 345/50R20 tires, when your speedometer says 60 mph you are really going

65.3 mph

345/50R20 is +2.72 inches (+8.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/40R20345/50R20Difference%
Overall diameter30.87"33.58"+2.72"+8.8%
Section width13.58"13.58"0.00"0.0%
Sidewall height5.43"6.79"+1.36"+25.0%
Circumference96.97"105.50"+8.53"+8.8%
Revolutions per mile653.4600.6−52.9−8.1%

Speedometer difference

If your vehicle came with 345/40R20 and you fit 345/50R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.8 mph+1.8 mph
30 mph32.6 mph+2.6 mph
40 mph43.5 mph+3.5 mph
50 mph54.4 mph+4.4 mph
60 mph65.3 mph+5.3 mph
65 mph70.7 mph+5.7 mph
70 mph76.2 mph+6.2 mph
75 mph81.6 mph+6.6 mph
80 mph87.0 mph+7.0 mph

Each size on its own

Questions

Which is bigger, 345/40R20 or 345/50R20?
345/50R20 is taller by 2.72 inches (33.58" vs 30.87"). 345/40R20 is the same width.
Can I replace 345/40R20 with 345/50R20?
It is not recommended without checking carefully. The diameter difference is +8.8%, 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 345/50R20 change my speedometer?
When your speedometer reads 60 mph with 345/50R20 fitted (calibrated for 345/40R20), your real speed is 65.3 mph. Your odometer will under-count miles by about 8.8%.
Will 345/50R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.36 inches of ground clearance at the axle.

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