235/65R18 vs 285/45R20

285/45R20 is 0.07 in (1.8 mm) taller than 235/65R18, a +0.24% diameter difference. With the speedometer set for 235/65R18, an indicated 60 mph is 60.1 mph on 285/45R20.

Is this a safe swap?

Diameter difference +0.24%: under 1%

By overall diameter alone, 285/45R20 is a near match for 235/65R18; 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.

235/65R18 vs 285/45R20 differences
Measurement	235/65R18	285/45R20	Difference	% diff
Overall diameter	30.03 in (762.7 mm)	30.10 in (764.5 mm)	+0.07 in (+1.8 mm)	+0.24%
Sidewall height	6.01 in (152.8 mm)	5.05 in (128.3 mm)	−0.96 in (−24.5 mm)	−16.04%
Section width	9.25 in (235.0 mm)	11.22 in (285.0 mm)	+1.97 in (+50.0 mm)	+21.28%
Circumference	94.33 in (2,396.1 mm)	94.56 in (2,401.7 mm)	+0.22 in (+5.7 mm)	+0.24%
Revolutions per mile	672	670	−2	−0.24%

True speed with the speedometer calibrated for 235/65R18
Indicated	Actual on 235/65R18	Actual on 285/45R20	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.2 mph	+0.2 mph

Ride-height change: 285/45R20 raises the vehicle by 0.04 in (0.9 mm) at the axle (half the +1.8 mm diameter difference).

285/45R20 is 0.07 in taller than 235/65R18 and 50 mm wider, a 0.24% difference in overall diameter.

With the speedometer calibrated for 235/65R18, an indicated 60 mph is actually 60.1 mph on 285/45R20, and revolutions per mile change from 672 to 670.

285/45R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 235/65R18.

Size pages

Other comparisons with 235/65R18

Other comparisons with 285/45R20

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