Tire Size Box

325/35R21 vs 315/40R21

315/40R21 compared with 325/35R21

+3.2% diameter, −0.4" width

315/40R21 is 30.92" tall vs 29.96". More than the 3% rule of thumb, so check carefully before switching.

30.0"21" rim4.5"12.8" wide
325/35R21 (solid) compared with 315/40R21 (dashed outline), drawn to scale

With 315/40R21 tires, when your speedometer says 60 mph you are really going

61.9 mph

315/40R21 is +0.96 inches (+3.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/35R21315/40R21Difference%
Overall diameter29.96"30.92"+0.96"+3.2%
Section width12.80"12.40"−0.39"−3.1%
Sidewall height4.48"4.96"+0.48"+10.8%
Circumference94.11"97.14"+3.03"+3.2%
Revolutions per mile673.2652.2−21.0−3.1%

Speedometer difference

If your vehicle came with 325/35R21 and you fit 315/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.6 mph+1.6 mph
60 mph61.9 mph+1.9 mph
65 mph67.1 mph+2.1 mph
70 mph72.3 mph+2.3 mph
75 mph77.4 mph+2.4 mph
80 mph82.6 mph+2.6 mph

Each size on its own

Questions

Which is bigger, 325/35R21 or 315/40R21?
315/40R21 is taller by 0.96 inches (30.92" vs 29.96"). 325/35R21 is wider by 0.39 inches.
Can I replace 325/35R21 with 315/40R21?
It is not recommended without checking carefully. The diameter difference is +3.2%, 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 315/40R21 change my speedometer?
When your speedometer reads 60 mph with 315/40R21 fitted (calibrated for 325/35R21), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.2%.
Will 315/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.48 inches of ground clearance at the axle.

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