Tire Size Box

355/40R21 vs 345/45R21

345/45R21 compared with 355/40R21

+3.2% diameter, −0.4" width

345/45R21 is 33.22" tall vs 32.18". More than the 3% rule of thumb, so check carefully before switching.

32.2"21" rim5.6"14.0" wide
355/40R21 (solid) compared with 345/45R21 (dashed outline), drawn to scale

With 345/45R21 tires, when your speedometer says 60 mph you are really going

61.9 mph

345/45R21 is +1.04 inches (+3.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/40R21345/45R21Difference%
Overall diameter32.18"33.22"+1.04"+3.2%
Section width13.98"13.58"−0.39"−2.8%
Sidewall height5.59"6.11"+0.52"+9.3%
Circumference101.10"104.38"+3.28"+3.2%
Revolutions per mile626.7607.0−19.7−3.1%

Speedometer difference

If your vehicle came with 355/40R21 and you fit 345/45R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.6 mph+1.6 mph
60 mph61.9 mph+1.9 mph
65 mph67.1 mph+2.1 mph
70 mph72.3 mph+2.3 mph
75 mph77.4 mph+2.4 mph
80 mph82.6 mph+2.6 mph

Each size on its own

Questions

Which is bigger, 355/40R21 or 345/45R21?
345/45R21 is taller by 1.04 inches (33.22" vs 32.18"). 355/40R21 is wider by 0.39 inches.
Can I replace 355/40R21 with 345/45R21?
It is not recommended without checking carefully. The diameter difference is +3.2%, 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 345/45R21 change my speedometer?
When your speedometer reads 60 mph with 345/45R21 fitted (calibrated for 355/40R21), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.2%.
Will 345/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.52 inches of ground clearance at the axle.

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