Tire Size Box

305/70R18 vs 325/70R18

325/70R18 compared with 305/70R18

+3.2% diameter, +0.8" width

325/70R18 is 35.91" tall vs 34.81". More than the 3% rule of thumb, so check carefully before switching.

34.8"18" rim8.4"12.0" wide
305/70R18 (solid) compared with 325/70R18 (dashed outline), drawn to scale

With 325/70R18 tires, when your speedometer says 60 mph you are really going

61.9 mph

325/70R18 is +1.10 inches (+3.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/70R18325/70R18Difference%
Overall diameter34.81"35.91"+1.10"+3.2%
Section width12.01"12.80"+0.79"+6.6%
Sidewall height8.41"8.96"+0.55"+6.6%
Circumference109.36"112.83"+3.46"+3.2%
Revolutions per mile579.4561.6−17.8−3.1%

Speedometer difference

If your vehicle came with 305/70R18 and you fit 325/70R18, 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 mph72.2 mph+2.2 mph
75 mph77.4 mph+2.4 mph
80 mph82.5 mph+2.5 mph

Each size on its own

Questions

Which is bigger, 305/70R18 or 325/70R18?
325/70R18 is taller by 1.10 inches (35.91" vs 34.81"). 325/70R18 is wider by 0.79 inches.
Can I replace 305/70R18 with 325/70R18?
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 325/70R18 change my speedometer?
When your speedometer reads 60 mph with 325/70R18 fitted (calibrated for 305/70R18), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.2%.
Will 325/70R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.55 inches of ground clearance at the axle.

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