Tire Size Box

255/55R20 vs 255/65R20

255/65R20 compared with 255/55R20

+6.5% diameter, 0.0" width

255/65R20 is 33.05" tall vs 31.04". More than the 3% rule of thumb, so check carefully before switching.

31.0"20" rim5.5"10.0" wide
255/55R20 (solid) compared with 255/65R20 (dashed outline), drawn to scale

With 255/65R20 tires, when your speedometer says 60 mph you are really going

63.9 mph

255/65R20 is +2.01 inches (+6.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/55R20255/65R20Difference%
Overall diameter31.04"33.05"+2.01"+6.5%
Section width10.04"10.04"0.00"0.0%
Sidewall height5.52"6.53"+1.00"+18.2%
Circumference97.53"103.83"+6.31"+6.5%
Revolutions per mile649.7610.2−39.5−6.1%

Speedometer difference

If your vehicle came with 255/55R20 and you fit 255/65R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.3 mph+1.3 mph
30 mph31.9 mph+1.9 mph
40 mph42.6 mph+2.6 mph
50 mph53.2 mph+3.2 mph
60 mph63.9 mph+3.9 mph
65 mph69.2 mph+4.2 mph
70 mph74.5 mph+4.5 mph
75 mph79.9 mph+4.9 mph
80 mph85.2 mph+5.2 mph

Each size on its own

Questions

Which is bigger, 255/55R20 or 255/65R20?
255/65R20 is taller by 2.01 inches (33.05" vs 31.04"). 255/55R20 is the same width.
Can I replace 255/55R20 with 255/65R20?
It is not recommended without checking carefully. The diameter difference is +6.5%, 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 255/65R20 change my speedometer?
When your speedometer reads 60 mph with 255/65R20 fitted (calibrated for 255/55R20), your real speed is 63.9 mph. Your odometer will under-count miles by about 6.5%.
Will 255/65R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.00 inches of ground clearance at the axle.

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