225/55R17 vs 285/40R18

285/40R18 is 0.23 in (5.9 mm) taller than 225/55R17, a +0.87% diameter difference. With the speedometer set for 225/55R17, an indicated 60 mph is 60.5 mph on 285/40R18.

Is this a safe swap?

Diameter difference +0.87%: under 1%

By overall diameter alone, 285/40R18 is a near match for 225/55R17; 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.

225/55R17 vs 285/40R18 differences
Measurement	225/55R17	285/40R18	Difference	% diff
Overall diameter	26.74 in (679.3 mm)	26.98 in (685.2 mm)	+0.23 in (+5.9 mm)	+0.87%
Sidewall height	4.87 in (123.8 mm)	4.49 in (114.0 mm)	−0.38 in (−9.7 mm)	−7.88%
Section width	8.86 in (225.0 mm)	11.22 in (285.0 mm)	+2.36 in (+60.0 mm)	+26.67%
Circumference	84.02 in (2,134.1 mm)	84.75 in (2,152.6 mm)	+0.73 in (+18.5 mm)	+0.87%
Revolutions per mile	754	748	−6	−0.86%

True speed with the speedometer calibrated for 225/55R17
Indicated	Actual on 225/55R17	Actual on 285/40R18	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.7 mph	+0.7 mph

Ride-height change: 285/40R18 raises the vehicle by 0.12 in (3.0 mm) at the axle (half the +5.9 mm diameter difference).

285/40R18 is 0.23 in taller than 225/55R17 and 60 mm wider, a 0.87% difference in overall diameter.

With the speedometer calibrated for 225/55R17, an indicated 60 mph is actually 60.5 mph on 285/40R18, and revolutions per mile change from 754 to 748.

285/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 225/55R17.

Size pages

Other comparisons with 225/55R17

Other comparisons with 285/40R18

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