Tire Size Box

355/45R20 vs 355/55R20

355/55R20 compared with 355/45R20

+8.6% diameter, 0.0" width

355/55R20 is 35.37" tall vs 32.58". More than the 3% rule of thumb, so check carefully before switching.

32.6"20" rim6.3"14.0" wide
355/45R20 (solid) compared with 355/55R20 (dashed outline), drawn to scale

With 355/55R20 tires, when your speedometer says 60 mph you are really going

65.1 mph

355/55R20 is +2.80 inches (+8.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/45R20355/55R20Difference%
Overall diameter32.58"35.37"+2.80"+8.6%
Section width13.98"13.98"0.00"0.0%
Sidewall height6.29"7.69"+1.40"+22.2%
Circumference102.35"111.13"+8.78"+8.6%
Revolutions per mile619.1570.1−48.9−7.9%

Speedometer difference

If your vehicle came with 355/45R20 and you fit 355/55R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.6 mph+2.6 mph
40 mph43.4 mph+3.4 mph
50 mph54.3 mph+4.3 mph
60 mph65.1 mph+5.1 mph
65 mph70.6 mph+5.6 mph
70 mph76.0 mph+6.0 mph
75 mph81.4 mph+6.4 mph
80 mph86.9 mph+6.9 mph

Each size on its own

Questions

Which is bigger, 355/45R20 or 355/55R20?
355/55R20 is taller by 2.80 inches (35.37" vs 32.58"). 355/45R20 is the same width.
Can I replace 355/45R20 with 355/55R20?
It is not recommended without checking carefully. The diameter difference is +8.6%, 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 355/55R20 change my speedometer?
When your speedometer reads 60 mph with 355/55R20 fitted (calibrated for 355/45R20), your real speed is 65.1 mph. Your odometer will under-count miles by about 8.6%.
Will 355/55R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.40 inches of ground clearance at the axle.

Other comparisons