Tire Size Box

255/65R18 vs 275/65R18

275/65R18 compared with 255/65R18

+3.3% diameter, +0.8" width

275/65R18 is 32.07" tall vs 31.05". More than the 3% rule of thumb, so check carefully before switching.

31.1"18" rim6.5"10.0" wide
255/65R18 (solid) compared with 275/65R18 (dashed outline), drawn to scale

With 275/65R18 tires, when your speedometer says 60 mph you are really going

62.0 mph

275/65R18 is +1.02 inches (+3.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/65R18275/65R18Difference%
Overall diameter31.05"32.07"+1.02"+3.3%
Section width10.04"10.83"+0.79"+7.8%
Sidewall height6.53"7.04"+0.51"+7.8%
Circumference97.55"100.77"+3.22"+3.3%
Revolutions per mile649.5628.8−20.7−3.2%

Speedometer difference

If your vehicle came with 255/65R18 and you fit 275/65R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.6 mph+1.6 mph
60 mph62.0 mph+2.0 mph
65 mph67.1 mph+2.1 mph
70 mph72.3 mph+2.3 mph
75 mph77.5 mph+2.5 mph
80 mph82.6 mph+2.6 mph

Each size on its own

Questions

Which is bigger, 255/65R18 or 275/65R18?
275/65R18 is taller by 1.02 inches (32.07" vs 31.05"). 275/65R18 is wider by 0.79 inches.
Can I replace 255/65R18 with 275/65R18?
It is not recommended without checking carefully. The diameter difference is +3.3%, 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 275/65R18 change my speedometer?
When your speedometer reads 60 mph with 275/65R18 fitted (calibrated for 255/65R18), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.3%.
Will 275/65R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.51 inches of ground clearance at the axle.

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