285/45R18 vs 235/45R20

235/45R20 is 0.23 in (5.8 mm) taller than 285/45R18, a +0.81% diameter difference. With the speedometer set for 285/45R18, an indicated 60 mph is 60.5 mph on 235/45R20.

Is this a safe swap?

Diameter difference +0.81%: under 1%

By overall diameter alone, 235/45R20 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 235/45R20 differences
Measurement	285/45R18	235/45R20	Difference	% diff
Overall diameter	28.10 in (713.7 mm)	28.33 in (719.5 mm)	+0.23 in (+5.8 mm)	+0.81%
Sidewall height	5.05 in (128.3 mm)	4.16 in (105.8 mm)	−0.89 in (−22.5 mm)	−17.54%
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)	88.99 in (2,260.4 mm)	+0.72 in (+18.2 mm)	+0.81%
Revolutions per mile	718	712	−6	−0.81%

True speed with the speedometer calibrated for 285/45R18
Indicated	Actual on 285/45R18	Actual on 235/45R20	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.6 mph	+0.6 mph

Ride-height change: 235/45R20 raises the vehicle by 0.11 in (2.9 mm) at the axle (half the +5.8 mm diameter difference).

235/45R20 is 0.23 in taller than 285/45R18 and 50 mm narrower, a 0.81% difference in overall diameter.

With the speedometer calibrated for 285/45R18, an indicated 60 mph is actually 60.5 mph on 235/45R20, and revolutions per mile change from 718 to 712.

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

Size pages

Other comparisons with 285/45R18

Other comparisons with 235/45R20

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