255/50R18 vs 235/55R18

235/55R18 is 0.14 in (3.5 mm) taller than 255/50R18, a +0.49% diameter difference. With the speedometer set for 255/50R18, an indicated 60 mph is 60.3 mph on 235/55R18.

Is this a safe swap?

Diameter difference +0.49%: under 1%

By overall diameter alone, 235/55R18 is a near match for 255/50R18; 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.

255/50R18 vs 235/55R18 differences
Measurement	255/50R18	235/55R18	Difference	% diff
Overall diameter	28.04 in (712.2 mm)	28.18 in (715.7 mm)	+0.14 in (+3.5 mm)	+0.49%
Sidewall height	5.02 in (127.5 mm)	5.09 in (129.3 mm)	+0.07 in (+1.8 mm)	+1.37%
Section width	10.04 in (255.0 mm)	9.25 in (235.0 mm)	−0.79 in (−20.0 mm)	−7.84%
Circumference	88.09 in (2,237.4 mm)	88.52 in (2,248.4 mm)	+0.43 in (+11.0 mm)	+0.49%
Revolutions per mile	719	716	−4	−0.49%

True speed with the speedometer calibrated for 255/50R18
Indicated	Actual on 255/50R18	Actual on 235/55R18	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.4 mph	+0.4 mph

Ride-height change: 235/55R18 raises the vehicle by 0.07 in (1.8 mm) at the axle (half the +3.5 mm diameter difference).

235/55R18 is 0.14 in taller than 255/50R18 and 20 mm narrower, a 0.49% difference in overall diameter.

With the speedometer calibrated for 255/50R18, an indicated 60 mph is actually 60.3 mph on 235/55R18, and revolutions per mile change from 719 to 716.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 255/50R18

Other comparisons with 235/55R18

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