275/40R18 vs 285/30R20

285/30R20 is 0.07 in (1.8 mm) taller than 275/40R18, a +0.27% diameter difference. With the speedometer set for 275/40R18, an indicated 60 mph is 60.2 mph on 285/30R20.

Is this a safe swap?

Diameter difference +0.27%: under 1%

By overall diameter alone, 285/30R20 is a near match for 275/40R18; 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.

275/40R18 vs 285/30R20 differences
Measurement	275/40R18	285/30R20	Difference	% diff
Overall diameter	26.66 in (677.2 mm)	26.73 in (679.0 mm)	+0.07 in (+1.8 mm)	+0.27%
Sidewall height	4.33 in (110.0 mm)	3.37 in (85.5 mm)	−0.96 in (−24.5 mm)	−22.27%
Section width	10.83 in (275.0 mm)	11.22 in (285.0 mm)	+0.39 in (+10.0 mm)	+3.64%
Circumference	83.76 in (2,127.5 mm)	83.98 in (2,133.1 mm)	+0.22 in (+5.7 mm)	+0.27%
Revolutions per mile	756	754	−2	−0.27%

True speed with the speedometer calibrated for 275/40R18
Indicated	Actual on 275/40R18	Actual on 285/30R20	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.2 mph	+0.2 mph
75 mph	75.0 mph	75.2 mph	+0.2 mph

Ride-height change: 285/30R20 raises the vehicle by 0.04 in (0.9 mm) at the axle (half the +1.8 mm diameter difference).

285/30R20 is 0.07 in taller than 275/40R18 and 10 mm wider, a 0.27% difference in overall diameter.

With the speedometer calibrated for 275/40R18, an indicated 60 mph is actually 60.2 mph on 285/30R20, and revolutions per mile change from 756 to 754.

285/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 275/40R18.

Size pages

Other comparisons with 275/40R18

Other comparisons with 285/30R20

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