Tire Size Box

355/40R20 vs 355/45R20

355/45R20 compared with 355/40R20

+4.5% diameter, 0.0" width

355/45R20 is 32.58" 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 355/45R20 (dashed outline), drawn to scale

With 355/45R20 tires, when your speedometer says 60 mph you are really going

62.7 mph

355/45R20 is +1.40 inches (+4.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/40R20355/45R20Difference%
Overall diameter31.18"32.58"+1.40"+4.5%
Section width13.98"13.98"0.00"0.0%
Sidewall height5.59"6.29"+0.70"+12.5%
Circumference97.96"102.35"+4.39"+4.5%
Revolutions per mile646.8619.1−27.7−4.3%

Speedometer difference

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

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.3 mph+1.3 mph
40 mph41.8 mph+1.8 mph
50 mph52.2 mph+2.2 mph
60 mph62.7 mph+2.7 mph
65 mph67.9 mph+2.9 mph
70 mph73.1 mph+3.1 mph
75 mph78.4 mph+3.4 mph
80 mph83.6 mph+3.6 mph

Each size on its own

Questions

Which is bigger, 355/40R20 or 355/45R20?
355/45R20 is taller by 1.40 inches (32.58" vs 31.18"). 355/40R20 is the same width.
Can I replace 355/40R20 with 355/45R20?
It is not recommended without checking carefully. The diameter difference is +4.5%, 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 355/45R20 change my speedometer?
When your speedometer reads 60 mph with 355/45R20 fitted (calibrated for 355/40R20), your real speed is 62.7 mph. Your odometer will under-count miles by about 4.5%.
Will 355/45R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.70 inches of ground clearance at the axle.

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