Tire Size Box

345/25R21 vs 345/30R21

345/30R21 compared with 345/25R21

+4.9% diameter, 0.0" width

345/30R21 is 29.15" tall vs 27.79". More than the 3% rule of thumb, so check carefully before switching.

27.8"21" rim3.4"13.6" wide
345/25R21 (solid) compared with 345/30R21 (dashed outline), drawn to scale

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

62.9 mph

345/30R21 is +1.36 inches (+4.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/25R21345/30R21Difference%
Overall diameter27.79"29.15"+1.36"+4.9%
Section width13.58"13.58"0.00"0.0%
Sidewall height3.40"4.07"+0.68"+20.0%
Circumference87.31"91.58"+4.27"+4.9%
Revolutions per mile725.7691.9−33.8−4.7%

Speedometer difference

If your vehicle came with 345/25R21 and you fit 345/30R21, 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 mph52.4 mph+2.4 mph
60 mph62.9 mph+2.9 mph
65 mph68.2 mph+3.2 mph
70 mph73.4 mph+3.4 mph
75 mph78.7 mph+3.7 mph
80 mph83.9 mph+3.9 mph

Each size on its own

Questions

Which is bigger, 345/25R21 or 345/30R21?
345/30R21 is taller by 1.36 inches (29.15" vs 27.79"). 345/25R21 is the same width.
Can I replace 345/25R21 with 345/30R21?
It is not recommended without checking carefully. The diameter difference is +4.9%, 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 345/30R21 change my speedometer?
When your speedometer reads 60 mph with 345/30R21 fitted (calibrated for 345/25R21), your real speed is 62.9 mph. Your odometer will under-count miles by about 4.9%.
Will 345/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.68 inches of ground clearance at the axle.

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