255/45R18 vs 275/45R18

275/45R18 is 0.71 in (18.0 mm) taller than 255/45R18, a +2.62% diameter difference. With the speedometer set for 255/45R18, an indicated 60 mph is 61.6 mph on 275/45R18.

Is this a safe swap?

Diameter difference +2.62%: within the ±3% guideline

By overall diameter alone, 275/45R18 falls inside the ±3% range commonly used as a guideline when replacing 255/45R18. 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.

255/45R18 vs 275/45R18 differences
Measurement	255/45R18	275/45R18	Difference	% diff
Overall diameter	27.04 in (686.7 mm)	27.74 in (704.7 mm)	+0.71 in (+18.0 mm)	+2.62%
Sidewall height	4.52 in (114.8 mm)	4.87 in (123.8 mm)	+0.35 in (+9.0 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	84.93 in (2,157.3 mm)	87.16 in (2,213.9 mm)	+2.23 in (+56.5 mm)	+2.62%
Revolutions per mile	746	727	−19	−2.55%

True speed with the speedometer calibrated for 255/45R18
Indicated	Actual on 255/45R18	Actual on 275/45R18	Difference
30 mph	30.0 mph	30.8 mph	+0.8 mph
60 mph	60.0 mph	61.6 mph	+1.6 mph
75 mph	75.0 mph	77.0 mph	+2.0 mph

Ride-height change: 275/45R18 raises the vehicle by 0.35 in (9.0 mm) at the axle (half the +18.0 mm diameter difference).

275/45R18 is 0.71 in taller than 255/45R18 and 20 mm wider, a 2.62% difference in overall diameter.

With the speedometer calibrated for 255/45R18, an indicated 60 mph is actually 61.6 mph on 275/45R18, and revolutions per mile change from 746 to 727.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 255/45R18

Other comparisons with 275/45R18

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