Tire Size Box

275/25R21 vs 255/30R21

255/30R21 compared with 275/25R21

+2.3% diameter, −0.8" width

255/30R21 is 27.02" tall vs 26.41". Within the 3% rule, so usually interchangeable.

26.4"21" rim2.7"10.8" wide
275/25R21 (solid) compared with 255/30R21 (dashed outline), drawn to scale

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

61.4 mph

255/30R21 is +0.61 inches (+2.3%) in overall diameter. That is within the 3% most shops and makers allow.

275/25R21255/30R21Difference%
Overall diameter26.41"27.02"+0.61"+2.3%
Section width10.83"10.04"−0.79"−7.3%
Sidewall height2.71"3.01"+0.31"+11.3%
Circumference82.98"84.90"+1.92"+2.3%
Revolutions per mile763.6746.3−17.2−2.3%

Speedometer difference

If your vehicle came with 275/25R21 and you fit 255/30R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.5 mph+0.5 mph
30 mph30.7 mph+0.7 mph
40 mph40.9 mph+0.9 mph
50 mph51.2 mph+1.2 mph
60 mph61.4 mph+1.4 mph
65 mph66.5 mph+1.5 mph
70 mph71.6 mph+1.6 mph
75 mph76.7 mph+1.7 mph
80 mph81.8 mph+1.8 mph

Each size on its own

Questions

Which is bigger, 275/25R21 or 255/30R21?
255/30R21 is taller by 0.61 inches (27.02" vs 26.41"). 275/25R21 is wider by 0.79 inches.
Can I replace 275/25R21 with 255/30R21?
Usually, yes. The diameter difference is +2.3%, 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 255/30R21 change my speedometer?
When your speedometer reads 60 mph with 255/30R21 fitted (calibrated for 275/25R21), your real speed is 61.4 mph. Your odometer will under-count miles by about 2.3%.
Will 255/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.31 inches of ground clearance at the axle.

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