Tire Size Box

355/25R21 vs 345/30R21

345/30R21 compared with 355/25R21

+4.1% diameter, −0.4" width

345/30R21 is 29.15" 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 345/30R21 (dashed outline), drawn to scale

With 345/30R21 tires, when your speedometer says 60 mph you are really going

62.5 mph

345/30R21 is +1.16 inches (+4.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/25R21345/30R21Difference%
Overall diameter27.99"29.15"+1.16"+4.1%
Section width13.98"13.58"−0.39"−2.8%
Sidewall height3.49"4.07"+0.58"+16.6%
Circumference87.93"91.58"+3.65"+4.1%
Revolutions per mile720.6691.9−28.7−4.0%

Speedometer difference

If your vehicle came with 355/25R21 and you fit 345/30R21, 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 mph41.7 mph+1.7 mph
50 mph52.1 mph+2.1 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, 355/25R21 or 345/30R21?
345/30R21 is taller by 1.16 inches (29.15" vs 27.99"). 355/25R21 is wider by 0.39 inches.
Can I replace 355/25R21 with 345/30R21?
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 345/30R21 change my speedometer?
When your speedometer reads 60 mph with 345/30R21 fitted (calibrated for 355/25R21), your real speed is 62.5 mph. Your odometer will under-count miles by about 4.1%.
Will 345/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.58 inches of ground clearance at the axle.

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