Tire Size Box

355/40R20 vs 345/45R20

345/45R20 compared with 355/40R20

+3.3% diameter, −0.4" width

345/45R20 is 32.22" tall vs 31.18". More than the 3% rule of thumb, so check carefully before switching.

31.2"20" rim5.6"14.0" wide
355/40R20 (solid) compared with 345/45R20 (dashed outline), drawn to scale

With 345/45R20 tires, when your speedometer says 60 mph you are really going

62.0 mph

345/45R20 is +1.04 inches (+3.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/40R20345/45R20Difference%
Overall diameter31.18"32.22"+1.04"+3.3%
Section width13.98"13.58"−0.39"−2.8%
Sidewall height5.59"6.11"+0.52"+9.3%
Circumference97.96"101.24"+3.28"+3.3%
Revolutions per mile646.8625.9−20.9−3.2%

Speedometer difference

If your vehicle came with 355/40R20 and you fit 345/45R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.7 mph+1.7 mph
60 mph62.0 mph+2.0 mph
65 mph67.2 mph+2.2 mph
70 mph72.3 mph+2.3 mph
75 mph77.5 mph+2.5 mph
80 mph82.7 mph+2.7 mph

Each size on its own

Questions

Which is bigger, 355/40R20 or 345/45R20?
345/45R20 is taller by 1.04 inches (32.22" vs 31.18"). 355/40R20 is wider by 0.39 inches.
Can I replace 355/40R20 with 345/45R20?
It is not recommended without checking carefully. The diameter difference is +3.3%, 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 345/45R20 change my speedometer?
When your speedometer reads 60 mph with 345/45R20 fitted (calibrated for 355/40R20), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.3%.
Will 345/45R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.52 inches of ground clearance at the axle.

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