Tire Size Box

285/25R21 vs 275/30R21

275/30R21 compared with 285/25R21

+3.3% diameter, −0.4" width

275/30R21 is 27.50" 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 275/30R21 (dashed outline), drawn to scale

With 275/30R21 tires, when your speedometer says 60 mph you are really going

62.0 mph

275/30R21 is +0.89 inches (+3.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/25R21275/30R21Difference%
Overall diameter26.61"27.50"+0.89"+3.3%
Section width11.22"10.83"−0.39"−3.5%
Sidewall height2.81"3.25"+0.44"+15.8%
Circumference83.60"86.38"+2.78"+3.3%
Revolutions per mile757.9733.5−24.4−3.2%

Speedometer difference

If your vehicle came with 285/25R21 and you fit 275/30R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.7 mph+1.7 mph
60 mph62.0 mph+2.0 mph
65 mph67.2 mph+2.2 mph
70 mph72.3 mph+2.3 mph
75 mph77.5 mph+2.5 mph
80 mph82.7 mph+2.7 mph

Each size on its own

Questions

Which is bigger, 285/25R21 or 275/30R21?
275/30R21 is taller by 0.89 inches (27.50" vs 26.61"). 285/25R21 is wider by 0.39 inches.
Can I replace 285/25R21 with 275/30R21?
It is not recommended without checking carefully. The diameter difference is +3.3%, 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 275/30R21 change my speedometer?
When your speedometer reads 60 mph with 275/30R21 fitted (calibrated for 285/25R21), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.3%.
Will 275/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

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