Tire Size Box

325/35R18 vs 315/40R18

315/40R18 compared with 325/35R18

+3.6% diameter, −0.4" width

315/40R18 is 27.92" tall vs 26.96". More than the 3% rule of thumb, so check carefully before switching.

27.0"18" rim4.5"12.8" wide
325/35R18 (solid) compared with 315/40R18 (dashed outline), drawn to scale

With 315/40R18 tires, when your speedometer says 60 mph you are really going

62.1 mph

315/40R18 is +0.96 inches (+3.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/35R18315/40R18Difference%
Overall diameter26.96"27.92"+0.96"+3.6%
Section width12.80"12.40"−0.39"−3.1%
Sidewall height4.48"4.96"+0.48"+10.8%
Circumference84.69"87.72"+3.03"+3.6%
Revolutions per mile748.2722.3−25.8−3.5%

Speedometer difference

If your vehicle came with 325/35R18 and you fit 315/40R18, 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 mph41.4 mph+1.4 mph
50 mph51.8 mph+1.8 mph
60 mph62.1 mph+2.1 mph
65 mph67.3 mph+2.3 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, 325/35R18 or 315/40R18?
315/40R18 is taller by 0.96 inches (27.92" vs 26.96"). 325/35R18 is wider by 0.39 inches.
Can I replace 325/35R18 with 315/40R18?
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 315/40R18 change my speedometer?
When your speedometer reads 60 mph with 315/40R18 fitted (calibrated for 325/35R18), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.6%.
Will 315/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.48 inches of ground clearance at the axle.

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