Tire Size Box

345/55R20 vs 35x12.50R20

35x12.50R20 compared with 345/55R20

+0.2% diameter, −1.1" width

35x12.50R20 is 35.00" tall vs 34.94". Within the 3% rule, so usually interchangeable.

34.9"20" rim7.5"13.6" wide
345/55R20 (solid) compared with 35x12.50R20 (dashed outline), drawn to scale

With 35x12.50R20 tires, when your speedometer says 60 mph you are really going

60.1 mph

35x12.50R20 is +0.06 inches (+0.2%) in overall diameter. That is within the 3% most shops and makers allow.

345/55R2035x12.50R20Difference%
Overall diameter34.94"35.00"+0.06"+0.2%
Section width13.58"12.50"−1.08"−8.0%
Sidewall height7.47"7.50"+0.03"+0.4%
Circumference109.77"109.96"+0.19"+0.2%
Revolutions per mile577.2576.2−1.0−0.2%

Speedometer difference

If your vehicle came with 345/55R20 and you fit 35x12.50R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.0 mph0.0 mph
30 mph30.1 mph+0.1 mph
40 mph40.1 mph+0.1 mph
50 mph50.1 mph+0.1 mph
60 mph60.1 mph+0.1 mph
65 mph65.1 mph+0.1 mph
70 mph70.1 mph+0.1 mph
75 mph75.1 mph+0.1 mph
80 mph80.1 mph+0.1 mph

Each size on its own

Questions

Which is bigger, 345/55R20 or 35x12.50R20?
35x12.50R20 is taller by 0.06 inches (35.00" vs 34.94"). 345/55R20 is wider by 1.08 inches.
Can I replace 345/55R20 with 35x12.50R20?
Usually, yes. The diameter difference is +0.2%, 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 35x12.50R20 change my speedometer?
When your speedometer reads 60 mph with 35x12.50R20 fitted (calibrated for 345/55R20), your real speed is 60.1 mph. Your odometer will under-count miles by about 0.2%.
Will 35x12.50R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.03 inches of ground clearance at the axle.

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