Tire Size Box

355/25R20 vs 345/30R20

345/30R20 compared with 355/25R20

+4.3% diameter, −0.4" width

345/30R20 is 28.15" tall vs 26.99". More than the 3% rule of thumb, so check carefully before switching.

27.0"20" rim3.5"14.0" wide
355/25R20 (solid) compared with 345/30R20 (dashed outline), drawn to scale

With 345/30R20 tires, when your speedometer says 60 mph you are really going

62.6 mph

345/30R20 is +1.16 inches (+4.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/25R20345/30R20Difference%
Overall diameter26.99"28.15"+1.16"+4.3%
Section width13.98"13.58"−0.39"−2.8%
Sidewall height3.49"4.07"+0.58"+16.6%
Circumference84.79"88.43"+3.65"+4.3%
Revolutions per mile747.3716.5−30.8−4.1%

Speedometer difference

If your vehicle came with 355/25R20 and you fit 345/30R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.3 mph+1.3 mph
40 mph41.7 mph+1.7 mph
50 mph52.2 mph+2.2 mph
60 mph62.6 mph+2.6 mph
65 mph67.8 mph+2.8 mph
70 mph73.0 mph+3.0 mph
75 mph78.2 mph+3.2 mph
80 mph83.4 mph+3.4 mph

Each size on its own

Questions

Which is bigger, 355/25R20 or 345/30R20?
345/30R20 is taller by 1.16 inches (28.15" vs 26.99"). 355/25R20 is wider by 0.39 inches.
Can I replace 355/25R20 with 345/30R20?
It is not recommended without checking carefully. The diameter difference is +4.3%, 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 345/30R20 change my speedometer?
When your speedometer reads 60 mph with 345/30R20 fitted (calibrated for 355/25R20), your real speed is 62.6 mph. Your odometer will under-count miles by about 4.3%.
Will 345/30R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.58 inches of ground clearance at the axle.

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