285/40R18 vs 225/40R20

225/40R20 is 0.11 in (2.8 mm) taller than 285/40R18, a +0.41% diameter difference. With the speedometer set for 285/40R18, an indicated 60 mph is 60.2 mph on 225/40R20.

Is this a safe swap?

Diameter difference +0.41%: under 1%

By overall diameter alone, 225/40R20 is a near match for 285/40R18; 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.

285/40R18 vs 225/40R20 differences
Measurement	285/40R18	225/40R20	Difference	% diff
Overall diameter	26.98 in (685.2 mm)	27.09 in (688.0 mm)	+0.11 in (+2.8 mm)	+0.41%
Sidewall height	4.49 in (114.0 mm)	3.54 in (90.0 mm)	−0.94 in (−24.0 mm)	−21.05%
Section width	11.22 in (285.0 mm)	8.86 in (225.0 mm)	−2.36 in (−60.0 mm)	−21.05%
Circumference	84.75 in (2,152.6 mm)	85.10 in (2,161.4 mm)	+0.35 in (+8.8 mm)	+0.41%
Revolutions per mile	748	745	−3	−0.41%

True speed with the speedometer calibrated for 285/40R18
Indicated	Actual on 285/40R18	Actual on 225/40R20	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	75.3 mph	+0.3 mph

Ride-height change: 225/40R20 raises the vehicle by 0.06 in (1.4 mm) at the axle (half the +2.8 mm diameter difference).

225/40R20 is 0.11 in taller than 285/40R18 and 60 mm narrower, a 0.41% difference in overall diameter.

With the speedometer calibrated for 285/40R18, an indicated 60 mph is actually 60.2 mph on 225/40R20, and revolutions per mile change from 748 to 745.

225/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 285/40R18.

Size pages

Other comparisons with 285/40R18

Other comparisons with 225/40R20

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