Tire Size Box

285/25R21 vs 285/30R21

285/30R21 compared with 285/25R21

+4.2% diameter, 0.0" width

285/30R21 is 27.73" tall vs 26.61". More than the 3% rule of thumb, so check carefully before switching.

26.6"21" rim2.8"11.2" wide
285/25R21 (solid) compared with 285/30R21 (dashed outline), drawn to scale

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

62.5 mph

285/30R21 is +1.12 inches (+4.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/25R21285/30R21Difference%
Overall diameter26.61"27.73"+1.12"+4.2%
Section width11.22"11.22"0.00"0.0%
Sidewall height2.81"3.37"+0.56"+20.0%
Circumference83.60"87.12"+3.53"+4.2%
Revolutions per mile757.9727.2−30.7−4.0%

Speedometer difference

If your vehicle came with 285/25R21 and you fit 285/30R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.3 mph+1.3 mph
40 mph41.7 mph+1.7 mph
50 mph52.1 mph+2.1 mph
60 mph62.5 mph+2.5 mph
65 mph67.7 mph+2.7 mph
70 mph73.0 mph+3.0 mph
75 mph78.2 mph+3.2 mph
80 mph83.4 mph+3.4 mph

Each size on its own

Questions

Which is bigger, 285/25R21 or 285/30R21?
285/30R21 is taller by 1.12 inches (27.73" vs 26.61"). 285/25R21 is the same width.
Can I replace 285/25R21 with 285/30R21?
It is not recommended without checking carefully. The diameter difference is +4.2%, 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 285/30R21 change my speedometer?
When your speedometer reads 60 mph with 285/30R21 fitted (calibrated for 285/25R21), your real speed is 62.5 mph. Your odometer will under-count miles by about 4.2%.
Will 285/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.56 inches of ground clearance at the axle.

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