Tire Size Box

325/25R20 vs 325/30R20

325/30R20 compared with 325/25R20

+4.8% diameter, 0.0" width

325/30R20 is 27.68" tall vs 26.40". More than the 3% rule of thumb, so check carefully before switching.

26.4"20" rim3.2"12.8" wide
325/25R20 (solid) compared with 325/30R20 (dashed outline), drawn to scale

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

62.9 mph

325/30R20 is +1.28 inches (+4.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/25R20325/30R20Difference%
Overall diameter26.40"27.68"+1.28"+4.8%
Section width12.80"12.80"0.00"0.0%
Sidewall height3.20"3.84"+0.64"+20.0%
Circumference82.93"86.95"+4.02"+4.8%
Revolutions per mile764.0728.7−35.3−4.6%

Speedometer difference

If your vehicle came with 325/25R20 and you fit 325/30R20, 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 mph41.9 mph+1.9 mph
50 mph52.4 mph+2.4 mph
60 mph62.9 mph+2.9 mph
65 mph68.2 mph+3.2 mph
70 mph73.4 mph+3.4 mph
75 mph78.6 mph+3.6 mph
80 mph83.9 mph+3.9 mph

Each size on its own

Questions

Which is bigger, 325/25R20 or 325/30R20?
325/30R20 is taller by 1.28 inches (27.68" vs 26.40"). 325/25R20 is the same width.
Can I replace 325/25R20 with 325/30R20?
It is not recommended without checking carefully. The diameter difference is +4.8%, 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 325/25R20), your real speed is 62.9 mph. Your odometer will under-count miles by about 4.8%.
Will 325/30R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.64 inches of ground clearance at the axle.

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