Tire Size Box

325/35R18 vs 325/40R18

325/40R18 compared with 325/35R18

+4.7% diameter, 0.0" width

325/40R18 is 28.24" tall vs 26.96". More than the 3% rule of thumb, so check carefully before switching.

27.0"18" rim4.5"12.8" wide
325/35R18 (solid) compared with 325/40R18 (dashed outline), drawn to scale

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

62.8 mph

325/40R18 is +1.28 inches (+4.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/35R18325/40R18Difference%
Overall diameter26.96"28.24"+1.28"+4.7%
Section width12.80"12.80"0.00"0.0%
Sidewall height4.48"5.12"+0.64"+14.3%
Circumference84.69"88.71"+4.02"+4.7%
Revolutions per mile748.2714.3−33.9−4.5%

Speedometer difference

If your vehicle came with 325/35R18 and you fit 325/40R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.4 mph+1.4 mph
40 mph41.9 mph+1.9 mph
50 mph52.4 mph+2.4 mph
60 mph62.8 mph+2.8 mph
65 mph68.1 mph+3.1 mph
70 mph73.3 mph+3.3 mph
75 mph78.6 mph+3.6 mph
80 mph83.8 mph+3.8 mph

Each size on its own

Questions

Which is bigger, 325/35R18 or 325/40R18?
325/40R18 is taller by 1.28 inches (28.24" vs 26.96"). 325/35R18 is the same width.
Can I replace 325/35R18 with 325/40R18?
It is not recommended without checking carefully. The diameter difference is +4.7%, 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 325/35R18), your real speed is 62.8 mph. Your odometer will under-count miles by about 4.7%.
Will 325/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.64 inches of ground clearance at the axle.

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