Tire Size Box

345/25R20 vs 345/30R20

345/30R20 compared with 345/25R20

+5.1% diameter, 0.0" width

345/30R20 is 28.15" 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 345/30R20 (dashed outline), drawn to scale

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

63.0 mph

345/30R20 is +1.36 inches (+5.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/25R20345/30R20Difference%
Overall diameter26.79"28.15"+1.36"+5.1%
Section width13.58"13.58"0.00"0.0%
Sidewall height3.40"4.07"+0.68"+20.0%
Circumference84.17"88.43"+4.27"+5.1%
Revolutions per mile752.8716.5−36.3−4.8%

Speedometer difference

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

Speedometer showsActual speedDifference
20 mph21.0 mph+1.0 mph
30 mph31.5 mph+1.5 mph
40 mph42.0 mph+2.0 mph
50 mph52.5 mph+2.5 mph
60 mph63.0 mph+3.0 mph
65 mph68.3 mph+3.3 mph
70 mph73.5 mph+3.5 mph
75 mph78.8 mph+3.8 mph
80 mph84.1 mph+4.1 mph

Each size on its own

Questions

Which is bigger, 345/25R20 or 345/30R20?
345/30R20 is taller by 1.36 inches (28.15" vs 26.79"). 345/25R20 is the same width.
Can I replace 345/25R20 with 345/30R20?
It is not recommended without checking carefully. The diameter difference is +5.1%, 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 345/30R20 change my speedometer?
When your speedometer reads 60 mph with 345/30R20 fitted (calibrated for 345/25R20), your real speed is 63.0 mph. Your odometer will under-count miles by about 5.1%.
Will 345/30R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.68 inches of ground clearance at the axle.

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