Tire Size Box

315/55R18 vs 325/55R18

325/55R18 compared with 315/55R18

+1.4% diameter, +0.4" width

325/55R18 is 32.07" tall vs 31.64". Within the 3% rule, so usually interchangeable.

31.6"18" rim6.8"12.4" wide
315/55R18 (solid) compared with 325/55R18 (dashed outline), drawn to scale

With 325/55R18 tires, when your speedometer says 60 mph you are really going

60.8 mph

325/55R18 is +0.43 inches (+1.4%) in overall diameter. That is within the 3% most shops and makers allow.

315/55R18325/55R18Difference%
Overall diameter31.64"32.07"+0.43"+1.4%
Section width12.40"12.80"+0.39"+3.2%
Sidewall height6.82"7.04"+0.22"+3.2%
Circumference99.41"100.77"+1.36"+1.4%
Revolutions per mile637.4628.8−8.6−1.4%

Speedometer difference

If your vehicle came with 315/55R18 and you fit 325/55R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.3 mph+0.3 mph
30 mph30.4 mph+0.4 mph
40 mph40.5 mph+0.5 mph
50 mph50.7 mph+0.7 mph
60 mph60.8 mph+0.8 mph
65 mph65.9 mph+0.9 mph
70 mph71.0 mph+1.0 mph
75 mph76.0 mph+1.0 mph
80 mph81.1 mph+1.1 mph

Each size on its own

Questions

Which is bigger, 315/55R18 or 325/55R18?
325/55R18 is taller by 0.43 inches (32.07" vs 31.64"). 325/55R18 is wider by 0.39 inches.
Can I replace 315/55R18 with 325/55R18?
Usually, yes. The diameter difference is +1.4%, 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 325/55R18 change my speedometer?
When your speedometer reads 60 mph with 325/55R18 fitted (calibrated for 315/55R18), your real speed is 60.8 mph. Your odometer will under-count miles by about 1.4%.
Will 325/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.22 inches of ground clearance at the axle.

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