Tire Size Box

305/35R21 vs 285/40R21

285/40R21 compared with 305/35R21

+1.9% diameter, −0.8" width

285/40R21 is 29.98" tall vs 29.41". Within the 3% rule, so usually interchangeable.

29.4"21" rim4.2"12.0" wide
305/35R21 (solid) compared with 285/40R21 (dashed outline), drawn to scale

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

61.2 mph

285/40R21 is +0.57 inches (+1.9%) in overall diameter. That is within the 3% most shops and makers allow.

305/35R21285/40R21Difference%
Overall diameter29.41"29.98"+0.57"+1.9%
Section width12.01"11.22"−0.79"−6.6%
Sidewall height4.20"4.49"+0.29"+6.8%
Circumference92.38"94.17"+1.79"+1.9%
Revolutions per mile685.9672.8−13.1−1.9%

Speedometer difference

If your vehicle came with 305/35R21 and you fit 285/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.4 mph+0.4 mph
30 mph30.6 mph+0.6 mph
40 mph40.8 mph+0.8 mph
50 mph51.0 mph+1.0 mph
60 mph61.2 mph+1.2 mph
65 mph66.3 mph+1.3 mph
70 mph71.4 mph+1.4 mph
75 mph76.5 mph+1.5 mph
80 mph81.6 mph+1.6 mph

Each size on its own

Questions

Which is bigger, 305/35R21 or 285/40R21?
285/40R21 is taller by 0.57 inches (29.98" vs 29.41"). 305/35R21 is wider by 0.79 inches.
Can I replace 305/35R21 with 285/40R21?
Usually, yes. The diameter difference is +1.9%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 285/40R21 change my speedometer?
When your speedometer reads 60 mph with 285/40R21 fitted (calibrated for 305/35R21), your real speed is 61.2 mph. Your odometer will under-count miles by about 1.9%.
Will 285/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.29 inches of ground clearance at the axle.

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