235/50R18 vs 255/50R18

255/50R18 is 0.79 in (20.0 mm) taller than 235/50R18, a +2.89% diameter difference. With the speedometer set for 235/50R18, an indicated 60 mph is 61.7 mph on 255/50R18.

Is this a safe swap?

Diameter difference +2.89%: within the ±3% guideline

By overall diameter alone, 255/50R18 falls inside the ±3% range commonly used as a guideline when replacing 235/50R18. 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.

235/50R18 vs 255/50R18 differences
Measurement	235/50R18	255/50R18	Difference	% diff
Overall diameter	27.25 in (692.2 mm)	28.04 in (712.2 mm)	+0.79 in (+20.0 mm)	+2.89%
Sidewall height	4.63 in (117.5 mm)	5.02 in (127.5 mm)	+0.39 in (+10.0 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	85.61 in (2,174.6 mm)	88.09 in (2,237.4 mm)	+2.47 in (+62.8 mm)	+2.89%
Revolutions per mile	740	719	−21	−2.81%

True speed with the speedometer calibrated for 235/50R18
Indicated	Actual on 235/50R18	Actual on 255/50R18	Difference
30 mph	30.0 mph	30.9 mph	+0.9 mph
60 mph	60.0 mph	61.7 mph	+1.7 mph
75 mph	75.0 mph	77.2 mph	+2.2 mph

Ride-height change: 255/50R18 raises the vehicle by 0.39 in (10.0 mm) at the axle (half the +20.0 mm diameter difference).

255/50R18 is 0.79 in taller than 235/50R18 and 20 mm wider, a 2.89% difference in overall diameter.

With the speedometer calibrated for 235/50R18, an indicated 60 mph is actually 61.7 mph on 255/50R18, and revolutions per mile change from 740 to 719.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 235/50R18

Other comparisons with 255/50R18

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