Tire Size Box

345/40R21 vs 355/40R21

355/40R21 compared with 345/40R21

+1.0% diameter, +0.4" width

355/40R21 is 32.18" tall vs 31.87". Within the 3% rule, so usually interchangeable.

31.9"21" rim5.4"13.6" wide
345/40R21 (solid) compared with 355/40R21 (dashed outline), drawn to scale

With 355/40R21 tires, when your speedometer says 60 mph you are really going

60.6 mph

355/40R21 is +0.31 inches (+1.0%) in overall diameter. That is within the 3% most shops and makers allow.

345/40R21355/40R21Difference%
Overall diameter31.87"32.18"+0.31"+1.0%
Section width13.58"13.98"+0.39"+2.9%
Sidewall height5.43"5.59"+0.16"+2.9%
Circumference100.11"101.10"+0.99"+1.0%
Revolutions per mile632.9626.7−6.2−1.0%

Speedometer difference

If your vehicle came with 345/40R21 and you fit 355/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.2 mph+0.2 mph
30 mph30.3 mph+0.3 mph
40 mph40.4 mph+0.4 mph
50 mph50.5 mph+0.5 mph
60 mph60.6 mph+0.6 mph
65 mph65.6 mph+0.6 mph
70 mph70.7 mph+0.7 mph
75 mph75.7 mph+0.7 mph
80 mph80.8 mph+0.8 mph

Each size on its own

Questions

Which is bigger, 345/40R21 or 355/40R21?
355/40R21 is taller by 0.31 inches (32.18" vs 31.87"). 355/40R21 is wider by 0.39 inches.
Can I replace 345/40R21 with 355/40R21?
Usually, yes. The diameter difference is +1.0%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 355/40R21 change my speedometer?
When your speedometer reads 60 mph with 355/40R21 fitted (calibrated for 345/40R21), your real speed is 60.6 mph. Your odometer will under-count miles by about 1.0%.
Will 355/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.16 inches of ground clearance at the axle.

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