Tire Size Box

325/25R21 vs 325/30R21

325/30R21 compared with 325/25R21

+4.7% diameter, 0.0" width

325/30R21 is 28.68" tall vs 27.40". More than the 3% rule of thumb, so check carefully before switching.

27.4"21" rim3.2"12.8" wide
325/25R21 (solid) compared with 325/30R21 (dashed outline), drawn to scale

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

62.8 mph

325/30R21 is +1.28 inches (+4.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/25R21325/30R21Difference%
Overall diameter27.40"28.68"+1.28"+4.7%
Section width12.80"12.80"0.00"0.0%
Sidewall height3.20"3.84"+0.64"+20.0%
Circumference86.07"90.09"+4.02"+4.7%
Revolutions per mile736.1703.3−32.8−4.5%

Speedometer difference

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

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.4 mph+1.4 mph
40 mph41.9 mph+1.9 mph
50 mph52.3 mph+2.3 mph
60 mph62.8 mph+2.8 mph
65 mph68.0 mph+3.0 mph
70 mph73.3 mph+3.3 mph
75 mph78.5 mph+3.5 mph
80 mph83.7 mph+3.7 mph

Each size on its own

Questions

Which is bigger, 325/25R21 or 325/30R21?
325/30R21 is taller by 1.28 inches (28.68" vs 27.40"). 325/25R21 is the same width.
Can I replace 325/25R21 with 325/30R21?
It is not recommended without checking carefully. The diameter difference is +4.7%, 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 325/25R21), your real speed is 62.8 mph. Your odometer will under-count miles by about 4.7%.
Will 325/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.64 inches of ground clearance at the axle.

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