Tire Size Box

285/35R20 vs 285/40R20

285/40R20 compared with 285/35R20

+4.0% diameter, 0.0" width

285/40R20 is 28.98" tall vs 27.85". More than the 3% rule of thumb, so check carefully before switching.

27.9"20" rim3.9"11.2" wide
285/35R20 (solid) compared with 285/40R20 (dashed outline), drawn to scale

With 285/40R20 tires, when your speedometer says 60 mph you are really going

62.4 mph

285/40R20 is +1.12 inches (+4.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/35R20285/40R20Difference%
Overall diameter27.85"28.98"+1.12"+4.0%
Section width11.22"11.22"0.00"0.0%
Sidewall height3.93"4.49"+0.56"+14.3%
Circumference87.51"91.03"+3.53"+4.0%
Revolutions per mile724.1696.0−28.0−3.9%

Speedometer difference

If your vehicle came with 285/35R20 and you fit 285/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.2 mph+1.2 mph
40 mph41.6 mph+1.6 mph
50 mph52.0 mph+2.0 mph
60 mph62.4 mph+2.4 mph
65 mph67.6 mph+2.6 mph
70 mph72.8 mph+2.8 mph
75 mph78.0 mph+3.0 mph
80 mph83.2 mph+3.2 mph

Each size on its own

Questions

Which is bigger, 285/35R20 or 285/40R20?
285/40R20 is taller by 1.12 inches (28.98" vs 27.85"). 285/35R20 is the same width.
Can I replace 285/35R20 with 285/40R20?
It is not recommended without checking carefully. The diameter difference is +4.0%, 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 285/40R20 change my speedometer?
When your speedometer reads 60 mph with 285/40R20 fitted (calibrated for 285/35R20), your real speed is 62.4 mph. Your odometer will under-count miles by about 4.0%.
Will 285/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.56 inches of ground clearance at the axle.

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