Tire Size Box

325/25R20 vs 305/30R20

305/30R20 compared with 325/25R20

+3.1% diameter, −0.8" width

305/30R20 is 27.20" 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 305/30R20 (dashed outline), drawn to scale

With 305/30R20 tires, when your speedometer says 60 mph you are really going

61.8 mph

305/30R20 is +0.81 inches (+3.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/25R20305/30R20Difference%
Overall diameter26.40"27.20"+0.81"+3.1%
Section width12.80"12.01"−0.79"−6.2%
Sidewall height3.20"3.60"+0.40"+12.6%
Circumference82.93"85.47"+2.54"+3.1%
Revolutions per mile764.0741.3−22.7−3.0%

Speedometer difference

If your vehicle came with 325/25R20 and you fit 305/30R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph30.9 mph+0.9 mph
40 mph41.2 mph+1.2 mph
50 mph51.5 mph+1.5 mph
60 mph61.8 mph+1.8 mph
65 mph67.0 mph+2.0 mph
70 mph72.1 mph+2.1 mph
75 mph77.3 mph+2.3 mph
80 mph82.4 mph+2.4 mph

Each size on its own

Questions

Which is bigger, 325/25R20 or 305/30R20?
305/30R20 is taller by 0.81 inches (27.20" vs 26.40"). 325/25R20 is wider by 0.79 inches.
Can I replace 325/25R20 with 305/30R20?
It is not recommended without checking carefully. The diameter difference is +3.1%, 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 305/30R20 change my speedometer?
When your speedometer reads 60 mph with 305/30R20 fitted (calibrated for 325/25R20), your real speed is 61.8 mph. Your odometer will under-count miles by about 3.1%.
Will 305/30R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.40 inches of ground clearance at the axle.

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