Tire Size Box

355/30R21 vs 345/35R21

345/35R21 compared with 355/30R21

+3.8% diameter, −0.4" width

345/35R21 is 30.51" tall vs 29.39". More than the 3% rule of thumb, so check carefully before switching.

29.4"21" rim4.2"14.0" wide
355/30R21 (solid) compared with 345/35R21 (dashed outline), drawn to scale

With 345/35R21 tires, when your speedometer says 60 mph you are really going

62.3 mph

345/35R21 is +1.12 inches (+3.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/30R21345/35R21Difference%
Overall diameter29.39"30.51"+1.12"+3.8%
Section width13.98"13.58"−0.39"−2.8%
Sidewall height4.19"4.75"+0.56"+13.4%
Circumference92.32"95.84"+3.53"+3.8%
Revolutions per mile686.3661.1−25.2−3.7%

Speedometer difference

If your vehicle came with 355/30R21 and you fit 345/35R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.1 mph+1.1 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, 355/30R21 or 345/35R21?
345/35R21 is taller by 1.12 inches (30.51" vs 29.39"). 355/30R21 is wider by 0.39 inches.
Can I replace 355/30R21 with 345/35R21?
It is not recommended without checking carefully. The diameter difference is +3.8%, 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 345/35R21 change my speedometer?
When your speedometer reads 60 mph with 345/35R21 fitted (calibrated for 355/30R21), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.8%.
Will 345/35R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.56 inches of ground clearance at the axle.

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