Tire Size Box

345/25R20 vs 325/30R20

325/30R20 compared with 345/25R20

+3.3% diameter, −0.8" width

325/30R20 is 27.68" tall vs 26.79". More than the 3% rule of thumb, so check carefully before switching.

26.8"20" rim3.4"13.6" wide
345/25R20 (solid) compared with 325/30R20 (dashed outline), drawn to scale

With 325/30R20 tires, when your speedometer says 60 mph you are really going

62.0 mph

325/30R20 is +0.89 inches (+3.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/25R20325/30R20Difference%
Overall diameter26.79"27.68"+0.89"+3.3%
Section width13.58"12.80"−0.79"−5.8%
Sidewall height3.40"3.84"+0.44"+13.0%
Circumference84.17"86.95"+2.78"+3.3%
Revolutions per mile752.8728.7−24.1−3.2%

Speedometer difference

If your vehicle came with 345/25R20 and you fit 325/30R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.7 mph+1.7 mph
60 mph62.0 mph+2.0 mph
65 mph67.1 mph+2.1 mph
70 mph72.3 mph+2.3 mph
75 mph77.5 mph+2.5 mph
80 mph82.6 mph+2.6 mph

Each size on its own

Questions

Which is bigger, 345/25R20 or 325/30R20?
325/30R20 is taller by 0.89 inches (27.68" vs 26.79"). 345/25R20 is wider by 0.79 inches.
Can I replace 345/25R20 with 325/30R20?
It is not recommended without checking carefully. The diameter difference is +3.3%, 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 325/30R20 change my speedometer?
When your speedometer reads 60 mph with 325/30R20 fitted (calibrated for 345/25R20), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.3%.
Will 325/30R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

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