Tire Size Box

255/25R21 vs 255/30R21

255/30R21 compared with 255/25R21

+3.9% diameter, 0.0" width

255/30R21 is 27.02" tall vs 26.02". More than the 3% rule of thumb, so check carefully before switching.

26.0"21" rim2.5"10.0" wide
255/25R21 (solid) compared with 255/30R21 (dashed outline), drawn to scale

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

62.3 mph

255/30R21 is +1.00 inches (+3.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/25R21255/30R21Difference%
Overall diameter26.02"27.02"+1.00"+3.9%
Section width10.04"10.04"0.00"0.0%
Sidewall height2.51"3.01"+0.50"+20.0%
Circumference81.74"84.90"+3.15"+3.9%
Revolutions per mile775.1746.3−28.8−3.7%

Speedometer difference

If your vehicle came with 255/25R21 and you fit 255/30R21, 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 mph41.5 mph+1.5 mph
50 mph51.9 mph+1.9 mph
60 mph62.3 mph+2.3 mph
65 mph67.5 mph+2.5 mph
70 mph72.7 mph+2.7 mph
75 mph77.9 mph+2.9 mph
80 mph83.1 mph+3.1 mph

Each size on its own

Questions

Which is bigger, 255/25R21 or 255/30R21?
255/30R21 is taller by 1.00 inches (27.02" vs 26.02"). 255/25R21 is the same width.
Can I replace 255/25R21 with 255/30R21?
It is not recommended without checking carefully. The diameter difference is +3.9%, 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 255/25R21), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.9%.
Will 255/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

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