Tire Size Box

285/30R20 vs 285/40R20

285/40R20 compared with 285/30R20

+8.4% diameter, 0.0" width

285/40R20 is 28.98" tall vs 26.73". More than the 3% rule of thumb, so check carefully before switching.

26.7"20" rim3.4"11.2" wide
285/30R20 (solid) compared with 285/40R20 (dashed outline), drawn to scale

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

65.0 mph

285/40R20 is +2.24 inches (+8.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/30R20285/40R20Difference%
Overall diameter26.73"28.98"+2.24"+8.4%
Section width11.22"11.22"0.00"0.0%
Sidewall height3.37"4.49"+1.12"+33.3%
Circumference83.98"91.03"+7.05"+8.4%
Revolutions per mile754.4696.0−58.4−7.7%

Speedometer difference

If your vehicle came with 285/30R20 and you fit 285/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.5 mph+2.5 mph
40 mph43.4 mph+3.4 mph
50 mph54.2 mph+4.2 mph
60 mph65.0 mph+5.0 mph
65 mph70.5 mph+5.5 mph
70 mph75.9 mph+5.9 mph
75 mph81.3 mph+6.3 mph
80 mph86.7 mph+6.7 mph

Each size on its own

Questions

Which is bigger, 285/30R20 or 285/40R20?
285/40R20 is taller by 2.24 inches (28.98" vs 26.73"). 285/30R20 is the same width.
Can I replace 285/30R20 with 285/40R20?
It is not recommended without checking carefully. The diameter difference is +8.4%, 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 285/30R20), your real speed is 65.0 mph. Your odometer will under-count miles by about 8.4%.
Will 285/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.12 inches of ground clearance at the axle.

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