235/45R18 vs 255/45R18

255/45R18 is 0.71 in (18.0 mm) taller than 235/45R18, a +2.69% diameter difference. With the speedometer set for 235/45R18, an indicated 60 mph is 61.6 mph on 255/45R18.

Is this a safe swap?

Diameter difference +2.69%: within the ±3% guideline

By overall diameter alone, 255/45R18 falls inside the ±3% range commonly used as a guideline when replacing 235/45R18. 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.

235/45R18 vs 255/45R18 differences
Measurement	235/45R18	255/45R18	Difference	% diff
Overall diameter	26.33 in (668.7 mm)	27.04 in (686.7 mm)	+0.71 in (+18.0 mm)	+2.69%
Sidewall height	4.16 in (105.8 mm)	4.52 in (114.8 mm)	+0.35 in (+9.0 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	82.71 in (2,100.8 mm)	84.93 in (2,157.3 mm)	+2.23 in (+56.5 mm)	+2.69%
Revolutions per mile	766	746	−20	−2.62%

True speed with the speedometer calibrated for 235/45R18
Indicated	Actual on 235/45R18	Actual on 255/45R18	Difference
30 mph	30.0 mph	30.8 mph	+0.8 mph
60 mph	60.0 mph	61.6 mph	+1.6 mph
75 mph	75.0 mph	77.0 mph	+2.0 mph

Ride-height change: 255/45R18 raises the vehicle by 0.35 in (9.0 mm) at the axle (half the +18.0 mm diameter difference).

255/45R18 is 0.71 in taller than 235/45R18 and 20 mm wider, a 2.69% difference in overall diameter.

With the speedometer calibrated for 235/45R18, an indicated 60 mph is actually 61.6 mph on 255/45R18, and revolutions per mile change from 766 to 746.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 235/45R18

Other comparisons with 255/45R18

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