285/60R18 vs 255/55R20

255/55R20 is 0.42 in (10.7 mm) shorter than 285/60R18, a −1.34% diameter difference. With the speedometer set for 285/60R18, an indicated 60 mph is 59.2 mph on 255/55R20.

Is this a safe swap?

Diameter difference −1.34%: within the ±3% guideline

By overall diameter alone, 255/55R20 falls inside the ±3% range commonly used as a guideline when replacing 285/60R18. 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.

285/60R18 vs 255/55R20 differences
Measurement	285/60R18	255/55R20	Difference	% diff
Overall diameter	31.46 in (799.2 mm)	31.04 in (788.5 mm)	−0.42 in (−10.7 mm)	−1.34%
Sidewall height	6.73 in (171.0 mm)	5.52 in (140.3 mm)	−1.21 in (−30.7 mm)	−17.98%
Section width	11.22 in (285.0 mm)	10.04 in (255.0 mm)	−1.18 in (−30.0 mm)	−10.53%
Circumference	98.85 in (2,510.8 mm)	97.53 in (2,477.1 mm)	−1.32 in (−33.6 mm)	−1.34%
Revolutions per mile	641	650	+9	+1.36%

True speed with the speedometer calibrated for 285/60R18
Indicated	Actual on 285/60R18	Actual on 255/55R20	Difference
30 mph	30.0 mph	29.6 mph	−0.4 mph
60 mph	60.0 mph	59.2 mph	−0.8 mph
75 mph	75.0 mph	74.0 mph	−1.0 mph

Ride-height change: 255/55R20 lowers the vehicle by 0.21 in (5.4 mm) at the axle (half the −10.7 mm diameter difference).

255/55R20 is 0.42 in shorter than 285/60R18 and 30 mm narrower, a 1.34% difference in overall diameter.

With the speedometer calibrated for 285/60R18, an indicated 60 mph is actually 59.2 mph on 255/55R20, and revolutions per mile change from 641 to 650.

255/55R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 285/60R18.

Size pages

Other comparisons with 285/60R18

Other comparisons with 255/55R20

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