Tire Size Box

285/30R21 vs 285/40R21

285/40R21 compared with 285/30R21

+8.1% diameter, 0.0" width

285/40R21 is 29.98" tall vs 27.73". More than the 3% rule of thumb, so check carefully before switching.

27.7"21" rim3.4"11.2" wide
285/30R21 (solid) compared with 285/40R21 (dashed outline), drawn to scale

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

64.9 mph

285/40R21 is +2.24 inches (+8.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/30R21285/40R21Difference%
Overall diameter27.73"29.98"+2.24"+8.1%
Section width11.22"11.22"0.00"0.0%
Sidewall height3.37"4.49"+1.12"+33.3%
Circumference87.12"94.17"+7.05"+8.1%
Revolutions per mile727.2672.8−54.4−7.5%

Speedometer difference

If your vehicle came with 285/30R21 and you fit 285/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.4 mph+2.4 mph
40 mph43.2 mph+3.2 mph
50 mph54.0 mph+4.0 mph
60 mph64.9 mph+4.9 mph
65 mph70.3 mph+5.3 mph
70 mph75.7 mph+5.7 mph
75 mph81.1 mph+6.1 mph
80 mph86.5 mph+6.5 mph

Each size on its own

Questions

Which is bigger, 285/30R21 or 285/40R21?
285/40R21 is taller by 2.24 inches (29.98" vs 27.73"). 285/30R21 is the same width.
Can I replace 285/30R21 with 285/40R21?
It is not recommended without checking carefully. The diameter difference is +8.1%, 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 285/40R21 change my speedometer?
When your speedometer reads 60 mph with 285/40R21 fitted (calibrated for 285/30R21), your real speed is 64.9 mph. Your odometer will under-count miles by about 8.1%.
Will 285/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.12 inches of ground clearance at the axle.

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