Tire Size Box

285/35R21 vs 285/40R21

285/40R21 compared with 285/35R21

+3.9% diameter, 0.0" width

285/40R21 is 29.98" tall vs 28.85". More than the 3% rule of thumb, so check carefully before switching.

28.9"21" rim3.9"11.2" wide
285/35R21 (solid) compared with 285/40R21 (dashed outline), drawn to scale

With 285/40R21 tires, when your speedometer says 60 mph you are really going

62.3 mph

285/40R21 is +1.12 inches (+3.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/35R21285/40R21Difference%
Overall diameter28.85"29.98"+1.12"+3.9%
Section width11.22"11.22"0.00"0.0%
Sidewall height3.93"4.49"+0.56"+14.3%
Circumference90.65"94.17"+3.53"+3.9%
Revolutions per mile699.0672.8−26.2−3.7%

Speedometer difference

If your vehicle came with 285/35R21 and you fit 285/40R21, 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 mph51.9 mph+1.9 mph
60 mph62.3 mph+2.3 mph
65 mph67.5 mph+2.5 mph
70 mph72.7 mph+2.7 mph
75 mph77.9 mph+2.9 mph
80 mph83.1 mph+3.1 mph

Each size on its own

Questions

Which is bigger, 285/35R21 or 285/40R21?
285/40R21 is taller by 1.12 inches (29.98" vs 28.85"). 285/35R21 is the same width.
Can I replace 285/35R21 with 285/40R21?
It is not recommended without checking carefully. The diameter difference is +3.9%, 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 285/40R21 change my speedometer?
When your speedometer reads 60 mph with 285/40R21 fitted (calibrated for 285/35R21), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.9%.
Will 285/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.56 inches of ground clearance at the axle.

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