Tire Size Box

265/25R21 vs 255/30R21

255/30R21 compared with 265/25R21

+3.1% diameter, −0.4" width

255/30R21 is 27.02" tall vs 26.22". More than the 3% rule of thumb, so check carefully before switching.

26.2"21" rim2.6"10.4" wide
265/25R21 (solid) compared with 255/30R21 (dashed outline), drawn to scale

With 255/30R21 tires, when your speedometer says 60 mph you are really going

61.8 mph

255/30R21 is +0.81 inches (+3.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

265/25R21255/30R21Difference%
Overall diameter26.22"27.02"+0.81"+3.1%
Section width10.43"10.04"−0.39"−3.8%
Sidewall height2.61"3.01"+0.40"+15.5%
Circumference82.36"84.90"+2.54"+3.1%
Revolutions per mile769.3746.3−23.0−3.0%

Speedometer difference

If your vehicle came with 265/25R21 and you fit 255/30R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph30.9 mph+0.9 mph
40 mph41.2 mph+1.2 mph
50 mph51.5 mph+1.5 mph
60 mph61.8 mph+1.8 mph
65 mph67.0 mph+2.0 mph
70 mph72.2 mph+2.2 mph
75 mph77.3 mph+2.3 mph
80 mph82.5 mph+2.5 mph

Each size on its own

Questions

Which is bigger, 265/25R21 or 255/30R21?
255/30R21 is taller by 0.81 inches (27.02" vs 26.22"). 265/25R21 is wider by 0.39 inches.
Can I replace 265/25R21 with 255/30R21?
It is not recommended without checking carefully. The diameter difference is +3.1%, 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 255/30R21 change my speedometer?
When your speedometer reads 60 mph with 255/30R21 fitted (calibrated for 265/25R21), your real speed is 61.8 mph. Your odometer will under-count miles by about 3.1%.
Will 255/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.40 inches of ground clearance at the axle.

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