275/55R18 vs 285/45R20

285/45R20 is 0.19 in (4.8 mm) taller than 275/55R18, a +0.63% diameter difference. With the speedometer set for 275/55R18, an indicated 60 mph is 60.4 mph on 285/45R20.

Is this a safe swap?

Diameter difference +0.63%: under 1%

By overall diameter alone, 285/45R20 is a near match for 275/55R18; 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.

275/55R18 vs 285/45R20 differences
Measurement	275/55R18	285/45R20	Difference	% diff
Overall diameter	29.91 in (759.7 mm)	30.10 in (764.5 mm)	+0.19 in (+4.8 mm)	+0.63%
Sidewall height	5.95 in (151.3 mm)	5.05 in (128.3 mm)	−0.91 in (−23.0 mm)	−15.21%
Section width	10.83 in (275.0 mm)	11.22 in (285.0 mm)	+0.39 in (+10.0 mm)	+3.64%
Circumference	93.96 in (2,386.7 mm)	94.56 in (2,401.7 mm)	+0.59 in (+15.1 mm)	+0.63%
Revolutions per mile	674	670	−4	−0.63%

True speed with the speedometer calibrated for 275/55R18
Indicated	Actual on 275/55R18	Actual on 285/45R20	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.4 mph	+0.4 mph
75 mph	75.0 mph	75.5 mph	+0.5 mph

Ride-height change: 285/45R20 raises the vehicle by 0.09 in (2.4 mm) at the axle (half the +4.8 mm diameter difference).

285/45R20 is 0.19 in taller than 275/55R18 and 10 mm wider, a 0.63% difference in overall diameter.

With the speedometer calibrated for 275/55R18, an indicated 60 mph is actually 60.4 mph on 285/45R20, and revolutions per mile change from 674 to 670.

285/45R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 275/55R18.

Size pages

Other comparisons with 275/55R18

Other comparisons with 285/45R20

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