285/45R18 vs 235/55R18

235/55R18 is 0.08 in (2.0 mm) taller than 285/45R18, a +0.28% diameter difference. With the speedometer set for 285/45R18, an indicated 60 mph is 60.2 mph on 235/55R18.

Is this a safe swap?

Diameter difference +0.28%: under 1%

By overall diameter alone, 235/55R18 is a near match for 285/45R18; 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.

285/45R18 vs 235/55R18 differences
Measurement	285/45R18	235/55R18	Difference	% diff
Overall diameter	28.10 in (713.7 mm)	28.18 in (715.7 mm)	+0.08 in (+2.0 mm)	+0.28%
Sidewall height	5.05 in (128.3 mm)	5.09 in (129.3 mm)	+0.04 in (+1.0 mm)	+0.78%
Section width	11.22 in (285.0 mm)	9.25 in (235.0 mm)	−1.97 in (−50.0 mm)	−17.54%
Circumference	88.27 in (2,242.2 mm)	88.52 in (2,248.4 mm)	+0.25 in (+6.3 mm)	+0.28%
Revolutions per mile	718	716	−2	−0.28%

True speed with the speedometer calibrated for 285/45R18
Indicated	Actual on 285/45R18	Actual on 235/55R18	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.2 mph	+0.2 mph
75 mph	75.0 mph	75.2 mph	+0.2 mph

Ride-height change: 235/55R18 raises the vehicle by 0.04 in (1.0 mm) at the axle (half the +2.0 mm diameter difference).

235/55R18 is 0.08 in taller than 285/45R18 and 50 mm narrower, a 0.28% difference in overall diameter.

With the speedometer calibrated for 285/45R18, an indicated 60 mph is actually 60.2 mph on 235/55R18, and revolutions per mile change from 718 to 716.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 285/45R18

Other comparisons with 235/55R18

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