Tire Size Box

255/55R18 vs 255/65R18

255/65R18 compared with 255/55R18

+6.9% diameter, 0.0" width

255/65R18 is 31.05" tall vs 29.04". More than the 3% rule of thumb, so check carefully before switching.

29.0"18" rim5.5"10.0" wide
255/55R18 (solid) compared with 255/65R18 (dashed outline), drawn to scale

With 255/65R18 tires, when your speedometer says 60 mph you are really going

64.1 mph

255/65R18 is +2.01 inches (+6.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/55R18255/65R18Difference%
Overall diameter29.04"31.05"+2.01"+6.9%
Section width10.04"10.04"0.00"0.0%
Sidewall height5.52"6.53"+1.00"+18.2%
Circumference91.24"97.55"+6.31"+6.9%
Revolutions per mile694.4649.5−44.9−6.5%

Speedometer difference

If your vehicle came with 255/55R18 and you fit 255/65R18, 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, 255/55R18 or 255/65R18?
255/65R18 is taller by 2.01 inches (31.05" vs 29.04"). 255/55R18 is the same width.
Can I replace 255/55R18 with 255/65R18?
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 255/65R18 change my speedometer?
When your speedometer reads 60 mph with 255/65R18 fitted (calibrated for 255/55R18), your real speed is 64.1 mph. Your odometer will under-count miles by about 6.9%.
Will 255/65R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.00 inches of ground clearance at the axle.

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