Tire Size Box

325/40R21 vs 315/45R21

315/45R21 compared with 325/40R21

+3.0% diameter, −0.4" width

315/45R21 is 32.16" tall vs 31.24". Within the 3% rule, so usually interchangeable.

31.2"21" rim5.1"12.8" wide
325/40R21 (solid) compared with 315/45R21 (dashed outline), drawn to scale

With 315/45R21 tires, when your speedometer says 60 mph you are really going

61.8 mph

315/45R21 is +0.93 inches (+3.0%) in overall diameter. That is within the 3% most shops and makers allow.

325/40R21315/45R21Difference%
Overall diameter31.24"32.16"+0.93"+3.0%
Section width12.80"12.40"−0.39"−3.1%
Sidewall height5.12"5.58"+0.46"+9.0%
Circumference98.13"101.04"+2.91"+3.0%
Revolutions per mile645.7627.1−18.6−2.9%

Speedometer difference

If your vehicle came with 325/40R21 and you fit 315/45R21, 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 mph66.9 mph+1.9 mph
70 mph72.1 mph+2.1 mph
75 mph77.2 mph+2.2 mph
80 mph82.4 mph+2.4 mph

Each size on its own

Questions

Which is bigger, 325/40R21 or 315/45R21?
315/45R21 is taller by 0.93 inches (32.16" vs 31.24"). 325/40R21 is wider by 0.39 inches.
Can I replace 325/40R21 with 315/45R21?
Usually, yes. The diameter difference is +3.0%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 315/45R21 change my speedometer?
When your speedometer reads 60 mph with 315/45R21 fitted (calibrated for 325/40R21), your real speed is 61.8 mph. Your odometer will under-count miles by about 3.0%.
Will 315/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

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