Tire Size Box

255/75R18 vs 275/75R18

275/75R18 compared with 255/75R18

+3.6% diameter, +0.8" width

275/75R18 is 34.24" tall vs 33.06". More than the 3% rule of thumb, so check carefully before switching.

33.1"18" rim7.5"10.0" wide
255/75R18 (solid) compared with 275/75R18 (dashed outline), drawn to scale

With 275/75R18 tires, when your speedometer says 60 mph you are really going

62.1 mph

275/75R18 is +1.18 inches (+3.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/75R18275/75R18Difference%
Overall diameter33.06"34.24"+1.18"+3.6%
Section width10.04"10.83"+0.79"+7.8%
Sidewall height7.53"8.12"+0.59"+7.8%
Circumference103.86"107.57"+3.71"+3.6%
Revolutions per mile610.1589.0−21.0−3.4%

Speedometer difference

If your vehicle came with 255/75R18 and you fit 275/75R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.4 mph+1.4 mph
50 mph51.8 mph+1.8 mph
60 mph62.1 mph+2.1 mph
65 mph67.3 mph+2.3 mph
70 mph72.5 mph+2.5 mph
75 mph77.7 mph+2.7 mph
80 mph82.9 mph+2.9 mph

Each size on its own

Questions

Which is bigger, 255/75R18 or 275/75R18?
275/75R18 is taller by 1.18 inches (34.24" vs 33.06"). 275/75R18 is wider by 0.79 inches.
Can I replace 255/75R18 with 275/75R18?
It is not recommended without checking carefully. The diameter difference is +3.6%, 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 275/75R18 change my speedometer?
When your speedometer reads 60 mph with 275/75R18 fitted (calibrated for 255/75R18), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.6%.
Will 275/75R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.59 inches of ground clearance at the axle.

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