Tire Size Box

355/25R20 vs 355/30R20

355/30R20 compared with 355/25R20

+5.2% diameter, 0.0" width

355/30R20 is 28.39" 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 355/30R20 (dashed outline), drawn to scale

With 355/30R20 tires, when your speedometer says 60 mph you are really going

63.1 mph

355/30R20 is +1.40 inches (+5.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/25R20355/30R20Difference%
Overall diameter26.99"28.39"+1.40"+5.2%
Section width13.98"13.98"0.00"0.0%
Sidewall height3.49"4.19"+0.70"+20.0%
Circumference84.79"89.18"+4.39"+5.2%
Revolutions per mile747.3710.5−36.8−4.9%

Speedometer difference

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

Speedometer showsActual speedDifference
20 mph21.0 mph+1.0 mph
30 mph31.6 mph+1.6 mph
40 mph42.1 mph+2.1 mph
50 mph52.6 mph+2.6 mph
60 mph63.1 mph+3.1 mph
65 mph68.4 mph+3.4 mph
70 mph73.6 mph+3.6 mph
75 mph78.9 mph+3.9 mph
80 mph84.1 mph+4.1 mph

Each size on its own

Questions

Which is bigger, 355/25R20 or 355/30R20?
355/30R20 is taller by 1.40 inches (28.39" vs 26.99"). 355/25R20 is the same width.
Can I replace 355/25R20 with 355/30R20?
It is not recommended without checking carefully. The diameter difference is +5.2%, 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 355/30R20 change my speedometer?
When your speedometer reads 60 mph with 355/30R20 fitted (calibrated for 355/25R20), your real speed is 63.1 mph. Your odometer will under-count miles by about 5.2%.
Will 355/30R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.70 inches of ground clearance at the axle.

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