Tire Size Box

275/25R21 vs 275/30R21

275/30R21 compared with 275/25R21

+4.1% diameter, 0.0" width

275/30R21 is 27.50" tall vs 26.41". More than the 3% rule of thumb, so check carefully before switching.

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

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

62.5 mph

275/30R21 is +1.08 inches (+4.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

275/25R21275/30R21Difference%
Overall diameter26.41"27.50"+1.08"+4.1%
Section width10.83"10.83"0.00"0.0%
Sidewall height2.71"3.25"+0.54"+20.0%
Circumference82.98"86.38"+3.40"+4.1%
Revolutions per mile763.6733.5−30.1−3.9%

Speedometer difference

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

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.2 mph+1.2 mph
40 mph41.6 mph+1.6 mph
50 mph52.0 mph+2.0 mph
60 mph62.5 mph+2.5 mph
65 mph67.7 mph+2.7 mph
70 mph72.9 mph+2.9 mph
75 mph78.1 mph+3.1 mph
80 mph83.3 mph+3.3 mph

Each size on its own

Questions

Which is bigger, 275/25R21 or 275/30R21?
275/30R21 is taller by 1.08 inches (27.50" vs 26.41"). 275/25R21 is the same width.
Can I replace 275/25R21 with 275/30R21?
It is not recommended without checking carefully. The diameter difference is +4.1%, 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 275/30R21 change my speedometer?
When your speedometer reads 60 mph with 275/30R21 fitted (calibrated for 275/25R21), your real speed is 62.5 mph. Your odometer will under-count miles by about 4.1%.
Will 275/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.54 inches of ground clearance at the axle.

Other comparisons