Tire Size Box

355/45R20 vs 345/50R20

345/50R20 compared with 355/45R20

+3.1% diameter, −0.4" width

345/50R20 is 33.58" tall vs 32.58". More than the 3% rule of thumb, so check carefully before switching.

32.6"20" rim6.3"14.0" wide
355/45R20 (solid) compared with 345/50R20 (dashed outline), drawn to scale

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

61.8 mph

345/50R20 is +1.00 inches (+3.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/45R20345/50R20Difference%
Overall diameter32.58"33.58"+1.00"+3.1%
Section width13.98"13.58"−0.39"−2.8%
Sidewall height6.29"6.79"+0.50"+8.0%
Circumference102.35"105.50"+3.15"+3.1%
Revolutions per mile619.1600.6−18.5−3.0%

Speedometer difference

If your vehicle came with 355/45R20 and you fit 345/50R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph30.9 mph+0.9 mph
40 mph41.2 mph+1.2 mph
50 mph51.5 mph+1.5 mph
60 mph61.8 mph+1.8 mph
65 mph67.0 mph+2.0 mph
70 mph72.2 mph+2.2 mph
75 mph77.3 mph+2.3 mph
80 mph82.5 mph+2.5 mph

Each size on its own

Questions

Which is bigger, 355/45R20 or 345/50R20?
345/50R20 is taller by 1.00 inches (33.58" vs 32.58"). 355/45R20 is wider by 0.39 inches.
Can I replace 355/45R20 with 345/50R20?
It is not recommended without checking carefully. The diameter difference is +3.1%, 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 355/45R20), your real speed is 61.8 mph. Your odometer will under-count miles by about 3.1%.
Will 345/50R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

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