255/55R18 vs 285/40R20

285/40R20 is 0.07 in (1.7 mm) shorter than 255/55R18, a −0.23% diameter difference. With the speedometer set for 255/55R18, an indicated 60 mph is 59.9 mph on 285/40R20.

Is this a safe swap?

Diameter difference −0.23%: under 1%

By overall diameter alone, 285/40R20 is a near match for 255/55R18; 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.

255/55R18 vs 285/40R20 differences
Measurement	255/55R18	285/40R20	Difference	% diff
Overall diameter	29.04 in (737.7 mm)	28.98 in (736.0 mm)	−0.07 in (−1.7 mm)	−0.23%
Sidewall height	5.52 in (140.3 mm)	4.49 in (114.0 mm)	−1.03 in (−26.2 mm)	−18.72%
Section width	10.04 in (255.0 mm)	11.22 in (285.0 mm)	+1.18 in (+30.0 mm)	+11.76%
Circumference	91.24 in (2,317.6 mm)	91.03 in (2,312.2 mm)	−0.21 in (−5.3 mm)	−0.23%
Revolutions per mile	694	696	+2	+0.23%

True speed with the speedometer calibrated for 255/55R18
Indicated	Actual on 255/55R18	Actual on 285/40R20	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.9 mph	−0.1 mph
75 mph	75.0 mph	74.8 mph	−0.2 mph

Ride-height change: 285/40R20 lowers the vehicle by 0.03 in (0.9 mm) at the axle (half the −1.7 mm diameter difference).

285/40R20 is 0.07 in shorter than 255/55R18 and 30 mm wider, a 0.23% difference in overall diameter.

With the speedometer calibrated for 255/55R18, an indicated 60 mph is actually 59.9 mph on 285/40R20, and revolutions per mile change from 694 to 696.

285/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/55R18.

Size pages

Other comparisons with 255/55R18

Other comparisons with 285/40R20

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