Tire Size Box

325/65R18 vs 325/70R18

325/70R18 compared with 325/65R18

+3.7% diameter, 0.0" width

325/70R18 is 35.91" tall vs 34.63". More than the 3% rule of thumb, so check carefully before switching.

34.6"18" rim8.3"12.8" wide
325/65R18 (solid) compared with 325/70R18 (dashed outline), drawn to scale

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

62.2 mph

325/70R18 is +1.28 inches (+3.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/65R18325/70R18Difference%
Overall diameter34.63"35.91"+1.28"+3.7%
Section width12.80"12.80"0.00"0.0%
Sidewall height8.32"8.96"+0.64"+7.7%
Circumference108.81"112.83"+4.02"+3.7%
Revolutions per mile582.3561.6−20.7−3.6%

Speedometer difference

If your vehicle came with 325/65R18 and you fit 325/70R18, 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 mph41.5 mph+1.5 mph
50 mph51.8 mph+1.8 mph
60 mph62.2 mph+2.2 mph
65 mph67.4 mph+2.4 mph
70 mph72.6 mph+2.6 mph
75 mph77.8 mph+2.8 mph
80 mph83.0 mph+3.0 mph

Each size on its own

Questions

Which is bigger, 325/65R18 or 325/70R18?
325/70R18 is taller by 1.28 inches (35.91" vs 34.63"). 325/65R18 is the same width.
Can I replace 325/65R18 with 325/70R18?
It is not recommended without checking carefully. The diameter difference is +3.7%, 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 325/65R18), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.7%.
Will 325/70R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.64 inches of ground clearance at the axle.

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