Tire Size Box

325/40R20 vs 315/45R20

315/45R20 compared with 325/40R20

+3.1% diameter, −0.4" width

315/45R20 is 31.16" tall vs 30.24". More than the 3% rule of thumb, so check carefully before switching.

30.2"20" rim5.1"12.8" wide
325/40R20 (solid) compared with 315/45R20 (dashed outline), drawn to scale

With 315/45R20 tires, when your speedometer says 60 mph you are really going

61.8 mph

315/45R20 is +0.93 inches (+3.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/40R20315/45R20Difference%
Overall diameter30.24"31.16"+0.93"+3.1%
Section width12.80"12.40"−0.39"−3.1%
Sidewall height5.12"5.58"+0.46"+9.0%
Circumference94.99"97.90"+2.91"+3.1%
Revolutions per mile667.0647.2−19.8−3.0%

Speedometer difference

If your vehicle came with 325/40R20 and you fit 315/45R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph30.9 mph+0.9 mph
40 mph41.2 mph+1.2 mph
50 mph51.5 mph+1.5 mph
60 mph61.8 mph+1.8 mph
65 mph67.0 mph+2.0 mph
70 mph72.1 mph+2.1 mph
75 mph77.3 mph+2.3 mph
80 mph82.4 mph+2.4 mph

Each size on its own

Questions

Which is bigger, 325/40R20 or 315/45R20?
315/45R20 is taller by 0.93 inches (31.16" vs 30.24"). 325/40R20 is wider by 0.39 inches.
Can I replace 325/40R20 with 315/45R20?
It is not recommended without checking carefully. The diameter difference is +3.1%, 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 315/45R20 change my speedometer?
When your speedometer reads 60 mph with 315/45R20 fitted (calibrated for 325/40R20), your real speed is 61.8 mph. Your odometer will under-count miles by about 3.1%.
Will 315/45R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

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