Tire Size Box

345/25R21 vs 325/30R21

325/30R21 compared with 345/25R21

+3.2% diameter, −0.8" width

325/30R21 is 28.68" tall vs 27.79". More than the 3% rule of thumb, so check carefully before switching.

27.8"21" rim3.4"13.6" wide
345/25R21 (solid) compared with 325/30R21 (dashed outline), drawn to scale

With 325/30R21 tires, when your speedometer says 60 mph you are really going

61.9 mph

325/30R21 is +0.89 inches (+3.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/25R21325/30R21Difference%
Overall diameter27.79"28.68"+0.89"+3.2%
Section width13.58"12.80"−0.79"−5.8%
Sidewall height3.40"3.84"+0.44"+13.0%
Circumference87.31"90.09"+2.78"+3.2%
Revolutions per mile725.7703.3−22.4−3.1%

Speedometer difference

If your vehicle came with 345/25R21 and you fit 325/30R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.6 mph+1.6 mph
60 mph61.9 mph+1.9 mph
65 mph67.1 mph+2.1 mph
70 mph72.2 mph+2.2 mph
75 mph77.4 mph+2.4 mph
80 mph82.5 mph+2.5 mph

Each size on its own

Questions

Which is bigger, 345/25R21 or 325/30R21?
325/30R21 is taller by 0.89 inches (28.68" vs 27.79"). 345/25R21 is wider by 0.79 inches.
Can I replace 345/25R21 with 325/30R21?
It is not recommended without checking carefully. The diameter difference is +3.2%, 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 325/30R21 change my speedometer?
When your speedometer reads 60 mph with 325/30R21 fitted (calibrated for 345/25R21), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.2%.
Will 325/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

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