Tire Size Box

285/25R20 vs 275/30R20

275/30R20 compared with 285/25R20

+3.5% diameter, −0.4" width

275/30R20 is 26.50" tall vs 25.61". More than the 3% rule of thumb, so check carefully before switching.

25.6"20" rim2.8"11.2" wide
285/25R20 (solid) compared with 275/30R20 (dashed outline), drawn to scale

With 275/30R20 tires, when your speedometer says 60 mph you are really going

62.1 mph

275/30R20 is +0.89 inches (+3.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/25R20275/30R20Difference%
Overall diameter25.61"26.50"+0.89"+3.5%
Section width11.22"10.83"−0.39"−3.5%
Sidewall height2.81"3.25"+0.44"+15.8%
Circumference80.46"83.24"+2.78"+3.5%
Revolutions per mile787.5761.2−26.3−3.3%

Speedometer difference

If your vehicle came with 285/25R20 and you fit 275/30R20, 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 mph41.4 mph+1.4 mph
50 mph51.7 mph+1.7 mph
60 mph62.1 mph+2.1 mph
65 mph67.2 mph+2.2 mph
70 mph72.4 mph+2.4 mph
75 mph77.6 mph+2.6 mph
80 mph82.8 mph+2.8 mph

Each size on its own

Questions

Which is bigger, 285/25R20 or 275/30R20?
275/30R20 is taller by 0.89 inches (26.50" vs 25.61"). 285/25R20 is wider by 0.39 inches.
Can I replace 285/25R20 with 275/30R20?
It is not recommended without checking carefully. The diameter difference is +3.5%, 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 275/30R20 change my speedometer?
When your speedometer reads 60 mph with 275/30R20 fitted (calibrated for 285/25R20), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.5%.
Will 275/30R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

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