Tire Size Box

305/65R18 vs 325/65R18

325/65R18 compared with 305/65R18

+3.0% diameter, +0.8" width

325/65R18 is 34.63" tall vs 33.61". More than the 3% rule of thumb, so check carefully before switching.

33.6"18" rim7.8"12.0" wide
305/65R18 (solid) compared with 325/65R18 (dashed outline), drawn to scale

With 325/65R18 tires, when your speedometer says 60 mph you are really going

61.8 mph

325/65R18 is +1.02 inches (+3.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/65R18325/65R18Difference%
Overall diameter33.61"34.63"+1.02"+3.0%
Section width12.01"12.80"+0.79"+6.6%
Sidewall height7.81"8.32"+0.51"+6.6%
Circumference105.59"108.81"+3.22"+3.0%
Revolutions per mile600.1582.3−17.7−3.0%

Speedometer difference

If your vehicle came with 305/65R18 and you fit 325/65R18, 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, 305/65R18 or 325/65R18?
325/65R18 is taller by 1.02 inches (34.63" vs 33.61"). 325/65R18 is wider by 0.79 inches.
Can I replace 305/65R18 with 325/65R18?
It is not recommended without checking carefully. The diameter difference is +3.0%, 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 325/65R18 change my speedometer?
When your speedometer reads 60 mph with 325/65R18 fitted (calibrated for 305/65R18), your real speed is 61.8 mph. Your odometer will under-count miles by about 3.0%.
Will 325/65R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.51 inches of ground clearance at the axle.

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