Tire Size Box

325/25R20 vs 315/30R20

315/30R20 compared with 325/25R20

+4.0% diameter, −0.4" width

315/30R20 is 27.44" 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 315/30R20 (dashed outline), drawn to scale

With 315/30R20 tires, when your speedometer says 60 mph you are really going

62.4 mph

315/30R20 is +1.04 inches (+4.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/25R20315/30R20Difference%
Overall diameter26.40"27.44"+1.04"+4.0%
Section width12.80"12.40"−0.39"−3.1%
Sidewall height3.20"3.72"+0.52"+16.3%
Circumference82.93"86.21"+3.28"+4.0%
Revolutions per mile764.0735.0−29.0−3.8%

Speedometer difference

If your vehicle came with 325/25R20 and you fit 315/30R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.2 mph+1.2 mph
40 mph41.6 mph+1.6 mph
50 mph52.0 mph+2.0 mph
60 mph62.4 mph+2.4 mph
65 mph67.6 mph+2.6 mph
70 mph72.8 mph+2.8 mph
75 mph78.0 mph+3.0 mph
80 mph83.2 mph+3.2 mph

Each size on its own

Questions

Which is bigger, 325/25R20 or 315/30R20?
315/30R20 is taller by 1.04 inches (27.44" vs 26.40"). 325/25R20 is wider by 0.39 inches.
Can I replace 325/25R20 with 315/30R20?
It is not recommended without checking carefully. The diameter difference is +4.0%, 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 315/30R20 change my speedometer?
When your speedometer reads 60 mph with 315/30R20 fitted (calibrated for 325/25R20), your real speed is 62.4 mph. Your odometer will under-count miles by about 4.0%.
Will 315/30R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.52 inches of ground clearance at the axle.

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