255/55R18 vs 275/55R18

275/55R18 is 0.87 in (22.0 mm) taller than 255/55R18, a +2.98% diameter difference. With the speedometer set for 255/55R18, an indicated 60 mph is 61.8 mph on 275/55R18.

Is this a safe swap?

Diameter difference +2.98%: within the ±3% guideline

By overall diameter alone, 275/55R18 falls inside the ±3% range commonly used as a guideline when replacing 255/55R18. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

Side profile and tread width, drawn to the same scale.

255/55R18 vs 275/55R18 differences
Measurement	255/55R18	275/55R18	Difference	% diff
Overall diameter	29.04 in (737.7 mm)	29.91 in (759.7 mm)	+0.87 in (+22.0 mm)	+2.98%
Sidewall height	5.52 in (140.3 mm)	5.95 in (151.3 mm)	+0.43 in (+11.0 mm)	+7.84%
Section width	10.04 in (255.0 mm)	10.83 in (275.0 mm)	+0.79 in (+20.0 mm)	+7.84%
Circumference	91.24 in (2,317.6 mm)	93.96 in (2,386.7 mm)	+2.72 in (+69.1 mm)	+2.98%
Revolutions per mile	694	674	−20	−2.90%

True speed with the speedometer calibrated for 255/55R18
Indicated	Actual on 255/55R18	Actual on 275/55R18	Difference
30 mph	30.0 mph	30.9 mph	+0.9 mph
60 mph	60.0 mph	61.8 mph	+1.8 mph
75 mph	75.0 mph	77.2 mph	+2.2 mph

Ride-height change: 275/55R18 raises the vehicle by 0.43 in (11.0 mm) at the axle (half the +22.0 mm diameter difference).

275/55R18 is 0.87 in taller than 255/55R18 and 20 mm wider, a 2.98% difference in overall diameter.

With the speedometer calibrated for 255/55R18, an indicated 60 mph is actually 61.8 mph on 275/55R18, and revolutions per mile change from 694 to 674.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 255/55R18

Other comparisons with 275/55R18

Source: Calculated from standard tire geometry formulas. Verified 2026-09-27. How these numbers are calculated.