Tire Size Box

325/55R18 vs 325/65R18

325/65R18 compared with 325/55R18

+8.0% diameter, 0.0" width

325/65R18 is 34.63" tall vs 32.07". More than the 3% rule of thumb, so check carefully before switching.

32.1"18" rim7.0"12.8" wide
325/55R18 (solid) compared with 325/65R18 (dashed outline), drawn to scale

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

64.8 mph

325/65R18 is +2.56 inches (+8.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/55R18325/65R18Difference%
Overall diameter32.07"34.63"+2.56"+8.0%
Section width12.80"12.80"0.00"0.0%
Sidewall height7.04"8.32"+1.28"+18.2%
Circumference100.77"108.81"+8.04"+8.0%
Revolutions per mile628.8582.3−46.5−7.4%

Speedometer difference

If your vehicle came with 325/55R18 and you fit 325/65R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.4 mph+2.4 mph
40 mph43.2 mph+3.2 mph
50 mph54.0 mph+4.0 mph
60 mph64.8 mph+4.8 mph
65 mph70.2 mph+5.2 mph
70 mph75.6 mph+5.6 mph
75 mph81.0 mph+6.0 mph
80 mph86.4 mph+6.4 mph

Each size on its own

Questions

Which is bigger, 325/55R18 or 325/65R18?
325/65R18 is taller by 2.56 inches (34.63" vs 32.07"). 325/55R18 is the same width.
Can I replace 325/55R18 with 325/65R18?
It is not recommended without checking carefully. The diameter difference is +8.0%, 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 325/55R18), your real speed is 64.8 mph. Your odometer will under-count miles by about 8.0%.
Will 325/65R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.28 inches of ground clearance at the axle.

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