285/75R18 vs 275/65R20

275/65R20 is 0.76 in (19.2 mm) shorter than 285/75R18, a −2.17% diameter difference. With the speedometer set for 285/75R18, an indicated 60 mph is 58.7 mph on 275/65R20.

Is this a safe swap?

Diameter difference −2.17%: within the ±3% guideline

By overall diameter alone, 275/65R20 falls inside the ±3% range commonly used as a guideline when replacing 285/75R18. 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.

285/75R18 vs 275/65R20 differences
Measurement	285/75R18	275/65R20	Difference	% diff
Overall diameter	34.83 in (884.7 mm)	34.07 in (865.5 mm)	−0.76 in (−19.2 mm)	−2.17%
Sidewall height	8.42 in (213.8 mm)	7.04 in (178.8 mm)	−1.38 in (−35.0 mm)	−16.37%
Section width	11.22 in (285.0 mm)	10.83 in (275.0 mm)	−0.39 in (−10.0 mm)	−3.51%
Circumference	109.42 in (2,779.4 mm)	107.05 in (2,719.0 mm)	−2.37 in (−60.3 mm)	−2.17%
Revolutions per mile	579	592	+13	+2.22%

True speed with the speedometer calibrated for 285/75R18
Indicated	Actual on 285/75R18	Actual on 275/65R20	Difference
30 mph	30.0 mph	29.3 mph	−0.7 mph
60 mph	60.0 mph	58.7 mph	−1.3 mph
75 mph	75.0 mph	73.4 mph	−1.6 mph

Ride-height change: 275/65R20 lowers the vehicle by 0.38 in (9.6 mm) at the axle (half the −19.2 mm diameter difference).

275/65R20 is 0.76 in shorter than 285/75R18 and 10 mm narrower, a 2.17% difference in overall diameter.

With the speedometer calibrated for 285/75R18, an indicated 60 mph is actually 58.7 mph on 275/65R20, and revolutions per mile change from 579 to 592.

275/65R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 285/75R18.

Size pages

Other comparisons with 285/75R18

Other comparisons with 275/65R20

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