Tire Size Box

325/35R20 vs 315/40R20

315/40R20 compared with 325/35R20

+3.3% diameter, −0.4" width

315/40R20 is 29.92" tall vs 28.96". More than the 3% rule of thumb, so check carefully before switching.

29.0"20" rim4.5"12.8" wide
325/35R20 (solid) compared with 315/40R20 (dashed outline), drawn to scale

With 315/40R20 tires, when your speedometer says 60 mph you are really going

62.0 mph

315/40R20 is +0.96 inches (+3.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/35R20315/40R20Difference%
Overall diameter28.96"29.92"+0.96"+3.3%
Section width12.80"12.40"−0.39"−3.1%
Sidewall height4.48"4.96"+0.48"+10.8%
Circumference90.97"94.00"+3.03"+3.3%
Revolutions per mile696.5674.0−22.5−3.2%

Speedometer difference

If your vehicle came with 325/35R20 and you fit 315/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.7 mph+1.7 mph
60 mph62.0 mph+2.0 mph
65 mph67.2 mph+2.2 mph
70 mph72.3 mph+2.3 mph
75 mph77.5 mph+2.5 mph
80 mph82.7 mph+2.7 mph

Each size on its own

Questions

Which is bigger, 325/35R20 or 315/40R20?
315/40R20 is taller by 0.96 inches (29.92" vs 28.96"). 325/35R20 is wider by 0.39 inches.
Can I replace 325/35R20 with 315/40R20?
It is not recommended without checking carefully. The diameter difference is +3.3%, 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 315/40R20 change my speedometer?
When your speedometer reads 60 mph with 315/40R20 fitted (calibrated for 325/35R20), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.3%.
Will 315/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.48 inches of ground clearance at the axle.

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