235/55R18 vs 255/55R18

255/55R18 is 0.87 in (22.0 mm) taller than 235/55R18, a +3.07% diameter difference. With the speedometer set for 235/55R18, an indicated 60 mph is 61.8 mph on 255/55R18.

Is this a safe swap?

Diameter difference +3.07%: outside the ±3% guideline

By overall diameter alone, 255/55R18 falls outside the ±3% range commonly used as a guideline when replacing 235/55R18; speedometer and ride-height changes are larger. 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.

235/55R18 vs 255/55R18 differences
Measurement	235/55R18	255/55R18	Difference	% diff
Overall diameter	28.18 in (715.7 mm)	29.04 in (737.7 mm)	+0.87 in (+22.0 mm)	+3.07%
Sidewall height	5.09 in (129.3 mm)	5.52 in (140.3 mm)	+0.43 in (+11.0 mm)	+8.51%
Section width	9.25 in (235.0 mm)	10.04 in (255.0 mm)	+0.79 in (+20.0 mm)	+8.51%
Circumference	88.52 in (2,248.4 mm)	91.24 in (2,317.6 mm)	+2.72 in (+69.1 mm)	+3.07%
Revolutions per mile	716	694	−21	−2.98%

True speed with the speedometer calibrated for 235/55R18
Indicated	Actual on 235/55R18	Actual on 255/55R18	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	77.3 mph	+2.3 mph

Ride-height change: 255/55R18 raises the vehicle by 0.43 in (11.0 mm) at the axle (half the +22.0 mm diameter difference).

255/55R18 is 0.87 in taller than 235/55R18 and 20 mm wider, a 3.07% difference in overall diameter.

With the speedometer calibrated for 235/55R18, an indicated 60 mph is actually 61.8 mph on 255/55R18, and revolutions per mile change from 716 to 694.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 235/55R18

Other comparisons with 255/55R18

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