255/45R18 vs 235/50R18

235/50R18 is 0.22 in (5.5 mm) taller than 255/45R18, a +0.80% diameter difference. With the speedometer set for 255/45R18, an indicated 60 mph is 60.5 mph on 235/50R18.

Is this a safe swap?

Diameter difference +0.80%: under 1%

By overall diameter alone, 235/50R18 is a near match for 255/45R18; 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.

255/45R18 vs 235/50R18 differences
Measurement	255/45R18	235/50R18	Difference	% diff
Overall diameter	27.04 in (686.7 mm)	27.25 in (692.2 mm)	+0.22 in (+5.5 mm)	+0.80%
Sidewall height	4.52 in (114.8 mm)	4.63 in (117.5 mm)	+0.11 in (+2.8 mm)	+2.40%
Section width	10.04 in (255.0 mm)	9.25 in (235.0 mm)	−0.79 in (−20.0 mm)	−7.84%
Circumference	84.93 in (2,157.3 mm)	85.61 in (2,174.6 mm)	+0.68 in (+17.3 mm)	+0.80%
Revolutions per mile	746	740	−6	−0.79%

True speed with the speedometer calibrated for 255/45R18
Indicated	Actual on 255/45R18	Actual on 235/50R18	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.6 mph	+0.6 mph

Ride-height change: 235/50R18 raises the vehicle by 0.11 in (2.8 mm) at the axle (half the +5.5 mm diameter difference).

235/50R18 is 0.22 in taller than 255/45R18 and 20 mm narrower, a 0.80% difference in overall diameter.

With the speedometer calibrated for 255/45R18, an indicated 60 mph is actually 60.5 mph on 235/50R18, and revolutions per mile change from 746 to 740.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 255/45R18

Other comparisons with 235/50R18

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