Tire Size Box

255/45R20 vs 255/55R20

255/55R20 compared with 255/45R20

+6.9% diameter, 0.0" width

255/55R20 is 31.04" tall vs 29.04". More than the 3% rule of thumb, so check carefully before switching.

29.0"20" rim4.5"10.0" wide
255/45R20 (solid) compared with 255/55R20 (dashed outline), drawn to scale

With 255/55R20 tires, when your speedometer says 60 mph you are really going

64.1 mph

255/55R20 is +2.01 inches (+6.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/45R20255/55R20Difference%
Overall diameter29.04"31.04"+2.01"+6.9%
Section width10.04"10.04"0.00"0.0%
Sidewall height4.52"5.52"+1.00"+22.2%
Circumference91.22"97.53"+6.31"+6.9%
Revolutions per mile694.6649.7−44.9−6.5%

Speedometer difference

If your vehicle came with 255/45R20 and you fit 255/55R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.4 mph+1.4 mph
30 mph32.1 mph+2.1 mph
40 mph42.8 mph+2.8 mph
50 mph53.5 mph+3.5 mph
60 mph64.1 mph+4.1 mph
65 mph69.5 mph+4.5 mph
70 mph74.8 mph+4.8 mph
75 mph80.2 mph+5.2 mph
80 mph85.5 mph+5.5 mph

Each size on its own

Questions

Which is bigger, 255/45R20 or 255/55R20?
255/55R20 is taller by 2.01 inches (31.04" vs 29.04"). 255/45R20 is the same width.
Can I replace 255/45R20 with 255/55R20?
It is not recommended without checking carefully. The diameter difference is +6.9%, 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 255/55R20 change my speedometer?
When your speedometer reads 60 mph with 255/55R20 fitted (calibrated for 255/45R20), your real speed is 64.1 mph. Your odometer will under-count miles by about 6.9%.
Will 255/55R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.00 inches of ground clearance at the axle.

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