235/55R17 vs 285/40R18

285/40R18 is 0.20 in (5.1 mm) shorter than 235/55R17, a −0.74% diameter difference. With the speedometer set for 235/55R17, an indicated 60 mph is 59.6 mph on 285/40R18.

Is this a safe swap?

Diameter difference −0.74%: under 1%

By overall diameter alone, 285/40R18 is a near match for 235/55R17; 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.

235/55R17 vs 285/40R18 differences
Measurement	235/55R17	285/40R18	Difference	% diff
Overall diameter	27.18 in (690.3 mm)	26.98 in (685.2 mm)	−0.20 in (−5.1 mm)	−0.74%
Sidewall height	5.09 in (129.3 mm)	4.49 in (114.0 mm)	−0.60 in (−15.2 mm)	−11.80%
Section width	9.25 in (235.0 mm)	11.22 in (285.0 mm)	+1.97 in (+50.0 mm)	+21.28%
Circumference	85.38 in (2,168.6 mm)	84.75 in (2,152.6 mm)	−0.63 in (−16.0 mm)	−0.74%
Revolutions per mile	742	748	+6	+0.74%

True speed with the speedometer calibrated for 235/55R17
Indicated	Actual on 235/55R17	Actual on 285/40R18	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.6 mph	−0.4 mph
75 mph	75.0 mph	74.4 mph	−0.6 mph

Ride-height change: 285/40R18 lowers the vehicle by 0.10 in (2.5 mm) at the axle (half the −5.1 mm diameter difference).

285/40R18 is 0.20 in shorter than 235/55R17 and 50 mm wider, a 0.74% difference in overall diameter.

With the speedometer calibrated for 235/55R17, an indicated 60 mph is actually 59.6 mph on 285/40R18, and revolutions per mile change from 742 to 748.

285/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 235/55R17.

Size pages

Other comparisons with 235/55R17

Other comparisons with 285/40R18

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