235/65R17 vs 255/55R18

255/55R18 is 0.02 in (0.4 mm) taller than 235/65R17, a +0.05% diameter difference. With the speedometer set for 235/65R17, an indicated 60 mph is 60.0 mph on 255/55R18.

Is this a safe swap?

Diameter difference +0.05%: under 1%

By overall diameter alone, 255/55R18 is a near match for 235/65R17; 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.

235/65R17 vs 255/55R18 differences
Measurement	235/65R17	255/55R18	Difference	% diff
Overall diameter	29.03 in (737.3 mm)	29.04 in (737.7 mm)	+0.02 in (+0.4 mm)	+0.05%
Sidewall height	6.01 in (152.8 mm)	5.52 in (140.3 mm)	−0.49 in (−12.5 mm)	−8.18%
Section width	9.25 in (235.0 mm)	10.04 in (255.0 mm)	+0.79 in (+20.0 mm)	+8.51%
Circumference	91.19 in (2,316.3 mm)	91.24 in (2,317.6 mm)	+0.05 in (+1.3 mm)	+0.05%
Revolutions per mile	695	694	0	−0.05%

True speed with the speedometer calibrated for 235/65R17
Indicated	Actual on 235/65R17	Actual on 255/55R18
30 mph	30.0 mph	30.0 mph
60 mph	60.0 mph	60.0 mph
75 mph	75.0 mph	75.0 mph

Ride-height change: 255/55R18 raises the vehicle by 0.01 in (0.2 mm) at the axle (half the +0.4 mm diameter difference).

255/55R18 is 0.02 in taller than 235/65R17 and 20 mm wider, a 0.05% difference in overall diameter.

With the speedometer calibrated for 235/65R17, an indicated 60 mph is actually 60.0 mph on 255/55R18, and revolutions per mile change from 695 to 694.

255/55R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 235/65R17.

Size pages

Other comparisons with 235/65R17

Other comparisons with 255/55R18

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