Tire Size Box

325/55R20 vs 345/55R20

345/55R20 compared with 325/55R20

+2.5% diameter, +0.8" width

345/55R20 is 34.94" tall vs 34.07". Within the 3% rule, so usually interchangeable.

34.1"20" rim7.0"12.8" wide
325/55R20 (solid) compared with 345/55R20 (dashed outline), drawn to scale

With 345/55R20 tires, when your speedometer says 60 mph you are really going

61.5 mph

345/55R20 is +0.87 inches (+2.5%) in overall diameter. That is within the 3% most shops and makers allow.

325/55R20345/55R20Difference%
Overall diameter34.07"34.94"+0.87"+2.5%
Section width12.80"13.58"+0.79"+6.2%
Sidewall height7.04"7.47"+0.43"+6.2%
Circumference107.05"109.77"+2.72"+2.5%
Revolutions per mile591.9577.2−14.7−2.5%

Speedometer difference

If your vehicle came with 325/55R20 and you fit 345/55R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.5 mph+0.5 mph
30 mph30.8 mph+0.8 mph
40 mph41.0 mph+1.0 mph
50 mph51.3 mph+1.3 mph
60 mph61.5 mph+1.5 mph
65 mph66.7 mph+1.7 mph
70 mph71.8 mph+1.8 mph
75 mph76.9 mph+1.9 mph
80 mph82.0 mph+2.0 mph

Each size on its own

Questions

Which is bigger, 325/55R20 or 345/55R20?
345/55R20 is taller by 0.87 inches (34.94" vs 34.07"). 345/55R20 is wider by 0.79 inches.
Can I replace 325/55R20 with 345/55R20?
Usually, yes. The diameter difference is +2.5%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 345/55R20 change my speedometer?
When your speedometer reads 60 mph with 345/55R20 fitted (calibrated for 325/55R20), your real speed is 61.5 mph. Your odometer will under-count miles by about 2.5%.
Will 345/55R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.43 inches of ground clearance at the axle.

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