Tire Size Box

285/35R18 vs 275/40R18

275/40R18 compared with 285/35R18

+3.1% diameter, −0.4" width

275/40R18 is 26.66" 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 275/40R18 (dashed outline), drawn to scale

With 275/40R18 tires, when your speedometer says 60 mph you are really going

61.9 mph

275/40R18 is +0.81 inches (+3.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/35R18275/40R18Difference%
Overall diameter25.85"26.66"+0.81"+3.1%
Section width11.22"10.83"−0.39"−3.5%
Sidewall height3.93"4.33"+0.40"+10.3%
Circumference81.22"83.76"+2.54"+3.1%
Revolutions per mile780.1756.5−23.6−3.0%

Speedometer difference

If your vehicle came with 285/35R18 and you fit 275/40R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph30.9 mph+0.9 mph
40 mph41.2 mph+1.2 mph
50 mph51.6 mph+1.6 mph
60 mph61.9 mph+1.9 mph
65 mph67.0 mph+2.0 mph
70 mph72.2 mph+2.2 mph
75 mph77.3 mph+2.3 mph
80 mph82.5 mph+2.5 mph

Each size on its own

Questions

Which is bigger, 285/35R18 or 275/40R18?
275/40R18 is taller by 0.81 inches (26.66" vs 25.85"). 285/35R18 is wider by 0.39 inches.
Can I replace 285/35R18 with 275/40R18?
It is not recommended without checking carefully. The diameter difference is +3.1%, 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 275/40R18 change my speedometer?
When your speedometer reads 60 mph with 275/40R18 fitted (calibrated for 285/35R18), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.1%.
Will 275/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.40 inches of ground clearance at the axle.

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