Tire Size Box

285/35R18 vs 285/40R18

285/40R18 compared with 285/35R18

+4.3% diameter, 0.0" width

285/40R18 is 26.98" tall vs 25.85". More than the 3% rule of thumb, so check carefully before switching.

25.9"18" rim3.9"11.2" wide
285/35R18 (solid) compared with 285/40R18 (dashed outline), drawn to scale

With 285/40R18 tires, when your speedometer says 60 mph you are really going

62.6 mph

285/40R18 is +1.12 inches (+4.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/35R18285/40R18Difference%
Overall diameter25.85"26.98"+1.12"+4.3%
Section width11.22"11.22"0.00"0.0%
Sidewall height3.93"4.49"+0.56"+14.3%
Circumference81.22"84.75"+3.53"+4.3%
Revolutions per mile780.1747.6−32.4−4.2%

Speedometer difference

If your vehicle came with 285/35R18 and you fit 285/40R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.3 mph+1.3 mph
40 mph41.7 mph+1.7 mph
50 mph52.2 mph+2.2 mph
60 mph62.6 mph+2.6 mph
65 mph67.8 mph+2.8 mph
70 mph73.0 mph+3.0 mph
75 mph78.3 mph+3.3 mph
80 mph83.5 mph+3.5 mph

Each size on its own

Questions

Which is bigger, 285/35R18 or 285/40R18?
285/40R18 is taller by 1.12 inches (26.98" vs 25.85"). 285/35R18 is the same width.
Can I replace 285/35R18 with 285/40R18?
It is not recommended without checking carefully. The diameter difference is +4.3%, 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 285/40R18 change my speedometer?
When your speedometer reads 60 mph with 285/40R18 fitted (calibrated for 285/35R18), your real speed is 62.6 mph. Your odometer will under-count miles by about 4.3%.
Will 285/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.56 inches of ground clearance at the axle.

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