255/65R17 vs 275/55R18

275/55R18 is 0.14 in (3.6 mm) shorter than 255/65R17, a −0.47% diameter difference. With the speedometer set for 255/65R17, an indicated 60 mph is 59.7 mph on 275/55R18.

Is this a safe swap?

Diameter difference −0.47%: under 1%

By overall diameter alone, 275/55R18 is a near match for 255/65R17; the speedometer changes by less than 1%. 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.

255/65R17 vs 275/55R18 differences
Measurement	255/65R17	275/55R18	Difference	% diff
Overall diameter	30.05 in (763.3 mm)	29.91 in (759.7 mm)	−0.14 in (−3.6 mm)	−0.47%
Sidewall height	6.53 in (165.8 mm)	5.95 in (151.3 mm)	−0.57 in (−14.5 mm)	−8.75%
Section width	10.04 in (255.0 mm)	10.83 in (275.0 mm)	+0.79 in (+20.0 mm)	+7.84%
Circumference	94.41 in (2,398.0 mm)	93.96 in (2,386.7 mm)	−0.45 in (−11.3 mm)	−0.47%
Revolutions per mile	671	674	+3	+0.47%

True speed with the speedometer calibrated for 255/65R17
Indicated	Actual on 255/65R17	Actual on 275/55R18	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.7 mph	−0.3 mph
75 mph	75.0 mph	74.6 mph	−0.4 mph

Ride-height change: 275/55R18 lowers the vehicle by 0.07 in (1.8 mm) at the axle (half the −3.6 mm diameter difference).

275/55R18 is 0.14 in shorter than 255/65R17 and 20 mm wider, a 0.47% difference in overall diameter.

With the speedometer calibrated for 255/65R17, an indicated 60 mph is actually 59.7 mph on 275/55R18, and revolutions per mile change from 671 to 674.

275/55R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 255/65R17.

Size pages

Other comparisons with 255/65R17

Other comparisons with 275/55R18

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