Tire Size Box

335/35R18 vs 325/40R18

325/40R18 compared with 335/35R18

+3.7% diameter, −0.4" width

325/40R18 is 28.24" tall vs 27.23". More than the 3% rule of thumb, so check carefully before switching.

27.2"18" rim4.6"13.2" wide
335/35R18 (solid) compared with 325/40R18 (dashed outline), drawn to scale

With 325/40R18 tires, when your speedometer says 60 mph you are really going

62.2 mph

325/40R18 is +1.00 inches (+3.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

335/35R18325/40R18Difference%
Overall diameter27.23"28.24"+1.00"+3.7%
Section width13.19"12.80"−0.39"−3.0%
Sidewall height4.62"5.12"+0.50"+10.9%
Circumference85.55"88.71"+3.15"+3.7%
Revolutions per mile740.6714.3−26.3−3.6%

Speedometer difference

If your vehicle came with 335/35R18 and you fit 325/40R18, 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 mph82.9 mph+2.9 mph

Each size on its own

Questions

Which is bigger, 335/35R18 or 325/40R18?
325/40R18 is taller by 1.00 inches (28.24" vs 27.23"). 335/35R18 is wider by 0.39 inches.
Can I replace 335/35R18 with 325/40R18?
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 325/40R18 change my speedometer?
When your speedometer reads 60 mph with 325/40R18 fitted (calibrated for 335/35R18), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.7%.
Will 325/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

Other comparisons