Tire Size Box

255/70R18 vs 275/70R18

275/70R18 compared with 255/70R18

+3.4% diameter, +0.8" width

275/70R18 is 33.16" tall vs 32.06". More than the 3% rule of thumb, so check carefully before switching.

32.1"18" rim7.0"10.0" wide
255/70R18 (solid) compared with 275/70R18 (dashed outline), drawn to scale

With 275/70R18 tires, when your speedometer says 60 mph you are really going

62.1 mph

275/70R18 is +1.10 inches (+3.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/70R18275/70R18Difference%
Overall diameter32.06"33.16"+1.10"+3.4%
Section width10.04"10.83"+0.79"+7.8%
Sidewall height7.03"7.58"+0.55"+7.8%
Circumference100.70"104.17"+3.46"+3.4%
Revolutions per mile629.2608.3−20.9−3.3%

Speedometer difference

If your vehicle came with 255/70R18 and you fit 275/70R18, 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 mph41.4 mph+1.4 mph
50 mph51.7 mph+1.7 mph
60 mph62.1 mph+2.1 mph
65 mph67.2 mph+2.2 mph
70 mph72.4 mph+2.4 mph
75 mph77.6 mph+2.6 mph
80 mph82.8 mph+2.8 mph

Each size on its own

Questions

Which is bigger, 255/70R18 or 275/70R18?
275/70R18 is taller by 1.10 inches (33.16" vs 32.06"). 275/70R18 is wider by 0.79 inches.
Can I replace 255/70R18 with 275/70R18?
It is not recommended without checking carefully. The diameter difference is +3.4%, 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 275/70R18 change my speedometer?
When your speedometer reads 60 mph with 275/70R18 fitted (calibrated for 255/70R18), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.4%.
Will 275/70R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.55 inches of ground clearance at the axle.

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