Tire Size Box

325/30R20 vs 315/35R20

315/35R20 compared with 325/30R20

+3.6% diameter, −0.4" width

315/35R20 is 28.68" tall vs 27.68". More than the 3% rule of thumb, so check carefully before switching.

27.7"20" rim3.8"12.8" wide
325/30R20 (solid) compared with 315/35R20 (dashed outline), drawn to scale

With 315/35R20 tires, when your speedometer says 60 mph you are really going

62.2 mph

315/35R20 is +1.00 inches (+3.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/30R20315/35R20Difference%
Overall diameter27.68"28.68"+1.00"+3.6%
Section width12.80"12.40"−0.39"−3.1%
Sidewall height3.84"4.34"+0.50"+13.1%
Circumference86.95"90.10"+3.15"+3.6%
Revolutions per mile728.7703.2−25.5−3.5%

Speedometer difference

If your vehicle came with 325/30R20 and you fit 315/35R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.8 mph+1.8 mph
60 mph62.2 mph+2.2 mph
65 mph67.4 mph+2.4 mph
70 mph72.5 mph+2.5 mph
75 mph77.7 mph+2.7 mph
80 mph82.9 mph+2.9 mph

Each size on its own

Questions

Which is bigger, 325/30R20 or 315/35R20?
315/35R20 is taller by 1.00 inches (28.68" vs 27.68"). 325/30R20 is wider by 0.39 inches.
Can I replace 325/30R20 with 315/35R20?
It is not recommended without checking carefully. The diameter difference is +3.6%, 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 315/35R20 change my speedometer?
When your speedometer reads 60 mph with 315/35R20 fitted (calibrated for 325/30R20), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.6%.
Will 315/35R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

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