Tire Size Box

355/25R21 vs 355/30R21

355/30R21 compared with 355/25R21

+5.0% diameter, 0.0" width

355/30R21 is 29.39" tall vs 27.99". More than the 3% rule of thumb, so check carefully before switching.

28.0"21" rim3.5"14.0" wide
355/25R21 (solid) compared with 355/30R21 (dashed outline), drawn to scale

With 355/30R21 tires, when your speedometer says 60 mph you are really going

63.0 mph

355/30R21 is +1.40 inches (+5.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/25R21355/30R21Difference%
Overall diameter27.99"29.39"+1.40"+5.0%
Section width13.98"13.98"0.00"0.0%
Sidewall height3.49"4.19"+0.70"+20.0%
Circumference87.93"92.32"+4.39"+5.0%
Revolutions per mile720.6686.3−34.3−4.8%

Speedometer difference

If your vehicle came with 355/25R21 and you fit 355/30R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.0 mph+1.0 mph
30 mph31.5 mph+1.5 mph
40 mph42.0 mph+2.0 mph
50 mph52.5 mph+2.5 mph
60 mph63.0 mph+3.0 mph
65 mph68.2 mph+3.2 mph
70 mph73.5 mph+3.5 mph
75 mph78.7 mph+3.7 mph
80 mph84.0 mph+4.0 mph

Each size on its own

Questions

Which is bigger, 355/25R21 or 355/30R21?
355/30R21 is taller by 1.40 inches (29.39" vs 27.99"). 355/25R21 is the same width.
Can I replace 355/25R21 with 355/30R21?
It is not recommended without checking carefully. The diameter difference is +5.0%, 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 355/30R21 change my speedometer?
When your speedometer reads 60 mph with 355/30R21 fitted (calibrated for 355/25R21), your real speed is 63.0 mph. Your odometer will under-count miles by about 5.0%.
Will 355/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.70 inches of ground clearance at the axle.

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