Tire Size Box

255/45R18 vs 275/45R18

275/45R18 compared with 255/45R18

+2.6% diameter, +0.8" width

275/45R18 is 27.74" tall vs 27.04". Within the 3% rule, so usually interchangeable.

27.0"18" rim4.5"10.0" wide
255/45R18 (solid) compared with 275/45R18 (dashed outline), drawn to scale

With 275/45R18 tires, when your speedometer says 60 mph you are really going

61.6 mph

275/45R18 is +0.71 inches (+2.6%) in overall diameter. That is within the 3% most shops and makers allow.

255/45R18275/45R18Difference%
Overall diameter27.04"27.74"+0.71"+2.6%
Section width10.04"10.83"+0.79"+7.8%
Sidewall height4.52"4.87"+0.35"+7.8%
Circumference84.93"87.16"+2.23"+2.6%
Revolutions per mile746.0726.9−19.1−2.6%

Speedometer difference

If your vehicle came with 255/45R18 and you fit 275/45R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.5 mph+0.5 mph
30 mph30.8 mph+0.8 mph
40 mph41.0 mph+1.0 mph
50 mph51.3 mph+1.3 mph
60 mph61.6 mph+1.6 mph
65 mph66.7 mph+1.7 mph
70 mph71.8 mph+1.8 mph
75 mph77.0 mph+2.0 mph
80 mph82.1 mph+2.1 mph

Each size on its own

Questions

Which is bigger, 255/45R18 or 275/45R18?
275/45R18 is taller by 0.71 inches (27.74" vs 27.04"). 275/45R18 is wider by 0.79 inches.
Can I replace 255/45R18 with 275/45R18?
Usually, yes. The diameter difference is +2.6%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 275/45R18 change my speedometer?
When your speedometer reads 60 mph with 275/45R18 fitted (calibrated for 255/45R18), your real speed is 61.6 mph. Your odometer will under-count miles by about 2.6%.
Will 275/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.35 inches of ground clearance at the axle.

Other comparisons