255/60R18 vs 285/45R20

285/45R20 is 0.05 in (1.3 mm) taller than 255/60R18, a +0.17% diameter difference. With the speedometer set for 255/60R18, an indicated 60 mph is 60.1 mph on 285/45R20.

Is this a safe swap?

Diameter difference +0.17%: under 1%

By overall diameter alone, 285/45R20 is a near match for 255/60R18; 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.

255/60R18 vs 285/45R20 differences
Measurement	255/60R18	285/45R20	Difference	% diff
Overall diameter	30.05 in (763.2 mm)	30.10 in (764.5 mm)	+0.05 in (+1.3 mm)	+0.17%
Sidewall height	6.02 in (153.0 mm)	5.05 in (128.3 mm)	−0.97 in (−24.7 mm)	−16.18%
Section width	10.04 in (255.0 mm)	11.22 in (285.0 mm)	+1.18 in (+30.0 mm)	+11.76%
Circumference	94.40 in (2,397.7 mm)	94.56 in (2,401.7 mm)	+0.16 in (+4.1 mm)	+0.17%
Revolutions per mile	671	670	−1	−0.17%

True speed with the speedometer calibrated for 255/60R18
Indicated	Actual on 255/60R18	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	75.1 mph	+0.1 mph

Ride-height change: 285/45R20 raises the vehicle by 0.03 in (0.6 mm) at the axle (half the +1.3 mm diameter difference).

285/45R20 is 0.05 in taller than 255/60R18 and 30 mm wider, a 0.17% difference in overall diameter.

With the speedometer calibrated for 255/60R18, an indicated 60 mph is actually 60.1 mph on 285/45R20, and revolutions per mile change from 671 to 670.

285/45R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/60R18.

Size pages

Other comparisons with 255/60R18

Other comparisons with 285/45R20

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