Tire Size Box

325/25R21 vs 315/30R21

315/30R21 compared with 325/25R21

+3.8% diameter, −0.4" width

315/30R21 is 28.44" 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 315/30R21 (dashed outline), drawn to scale

With 315/30R21 tires, when your speedometer says 60 mph you are really going

62.3 mph

315/30R21 is +1.04 inches (+3.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/25R21315/30R21Difference%
Overall diameter27.40"28.44"+1.04"+3.8%
Section width12.80"12.40"−0.39"−3.1%
Sidewall height3.20"3.72"+0.52"+16.3%
Circumference86.07"89.35"+3.28"+3.8%
Revolutions per mile736.1709.1−27.0−3.7%

Speedometer difference

If your vehicle came with 325/25R21 and you fit 315/30R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.9 mph+1.9 mph
60 mph62.3 mph+2.3 mph
65 mph67.5 mph+2.5 mph
70 mph72.7 mph+2.7 mph
75 mph77.9 mph+2.9 mph
80 mph83.0 mph+3.0 mph

Each size on its own

Questions

Which is bigger, 325/25R21 or 315/30R21?
315/30R21 is taller by 1.04 inches (28.44" vs 27.40"). 325/25R21 is wider by 0.39 inches.
Can I replace 325/25R21 with 315/30R21?
It is not recommended without checking carefully. The diameter difference is +3.8%, 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 315/30R21 change my speedometer?
When your speedometer reads 60 mph with 315/30R21 fitted (calibrated for 325/25R21), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.8%.
Will 315/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.52 inches of ground clearance at the axle.

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