Tire Size Box

245/40R20 vs 285/30R21

285/30R21 compared with 245/40R20

+0.1% diameter, +1.6" width

285/30R21 is 27.73" tall vs 27.72". Within the 3% rule, so usually interchangeable.

27.7"20" rim3.9"9.6" wide
245/40R20 (solid) compared with 285/30R21 (dashed outline), drawn to scale

With 285/30R21 tires, when your speedometer says 60 mph you are really going

60.0 mph

285/30R21 is +0.02 inches (+0.1%) in overall diameter. That is within the 3% most shops and makers allow.

245/40R20285/30R21Difference%
Overall diameter27.72"27.73"+0.02"+0.1%
Section width9.65"11.22"+1.57"+16.3%
Sidewall height3.86"3.37"−0.49"−12.8%
Circumference87.07"87.12"+0.05"+0.1%
Revolutions per mile727.7727.2−0.4−0.1%

Speedometer difference

If your vehicle came with 245/40R20 and you fit 285/30R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.0 mph0.0 mph
30 mph30.0 mph0.0 mph
40 mph40.0 mph0.0 mph
50 mph50.0 mph0.0 mph
60 mph60.0 mph0.0 mph
65 mph65.0 mph0.0 mph
70 mph70.0 mph0.0 mph
75 mph75.0 mph0.0 mph
80 mph80.0 mph0.0 mph

Each size on its own

Questions

Which is bigger, 245/40R20 or 285/30R21?
285/30R21 is taller by 0.02 inches (27.73" vs 27.72"). 285/30R21 is wider by 1.57 inches.
Can I replace 245/40R20 with 285/30R21?
Usually, yes. The diameter difference is +0.1%, 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 285/30R21 change my speedometer?
When your speedometer reads 60 mph with 285/30R21 fitted (calibrated for 245/40R20), your real speed is 60.0 mph. Your odometer will under-count miles by about 0.1%.
Will 285/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.01 inches of ground clearance at the axle.

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