Tire Size Box

345/45R21 vs 345/55R21

345/55R21 compared with 345/45R21

+8.2% diameter, 0.0" width

345/55R21 is 35.94" tall vs 33.22". More than the 3% rule of thumb, so check carefully before switching.

33.2"21" rim6.1"13.6" wide
345/45R21 (solid) compared with 345/55R21 (dashed outline), drawn to scale

With 345/55R21 tires, when your speedometer says 60 mph you are really going

64.9 mph

345/55R21 is +2.72 inches (+8.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/45R21345/55R21Difference%
Overall diameter33.22"35.94"+2.72"+8.2%
Section width13.58"13.58"0.00"0.0%
Sidewall height6.11"7.47"+1.36"+22.2%
Circumference104.38"112.91"+8.53"+8.2%
Revolutions per mile607.0561.1−45.9−7.6%

Speedometer difference

If your vehicle came with 345/45R21 and you fit 345/55R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.5 mph+2.5 mph
40 mph43.3 mph+3.3 mph
50 mph54.1 mph+4.1 mph
60 mph64.9 mph+4.9 mph
65 mph70.3 mph+5.3 mph
70 mph75.7 mph+5.7 mph
75 mph81.1 mph+6.1 mph
80 mph86.5 mph+6.5 mph

Each size on its own

Questions

Which is bigger, 345/45R21 or 345/55R21?
345/55R21 is taller by 2.72 inches (35.94" vs 33.22"). 345/45R21 is the same width.
Can I replace 345/45R21 with 345/55R21?
It is not recommended without checking carefully. The diameter difference is +8.2%, 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 345/55R21 change my speedometer?
When your speedometer reads 60 mph with 345/55R21 fitted (calibrated for 345/45R21), your real speed is 64.9 mph. Your odometer will under-count miles by about 8.2%.
Will 345/55R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.36 inches of ground clearance at the axle.

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