285/40R18 vs 285/30R20

285/30R20 is 0.24 in (6.2 mm) shorter than 285/40R18, a −0.90% diameter difference. With the speedometer set for 285/40R18, an indicated 60 mph is 59.5 mph on 285/30R20.

Is this a safe swap?

Diameter difference −0.90%: under 1%

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

285/40R18 vs 285/30R20 differences
Measurement	285/40R18	285/30R20	Difference	% diff
Overall diameter	26.98 in (685.2 mm)	26.73 in (679.0 mm)	−0.24 in (−6.2 mm)	−0.90%
Sidewall height	4.49 in (114.0 mm)	3.37 in (85.5 mm)	−1.12 in (−28.5 mm)	−25.00%
Section width	11.22 in (285.0 mm)	11.22 in (285.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	84.75 in (2,152.6 mm)	83.98 in (2,133.1 mm)	−0.77 in (−19.5 mm)	−0.90%
Revolutions per mile	748	754	+7	+0.91%

True speed with the speedometer calibrated for 285/40R18
Indicated	Actual on 285/40R18	Actual on 285/30R20	Difference
30 mph	30.0 mph	29.7 mph	−0.3 mph
60 mph	60.0 mph	59.5 mph	−0.5 mph
75 mph	75.0 mph	74.3 mph	−0.7 mph

Ride-height change: 285/30R20 lowers the vehicle by 0.12 in (3.1 mm) at the axle (half the −6.2 mm diameter difference).

285/30R20 is 0.24 in shorter than 285/40R18 and the same width, a 0.90% difference in overall diameter.

With the speedometer calibrated for 285/40R18, an indicated 60 mph is actually 59.5 mph on 285/30R20, and revolutions per mile change from 748 to 754.

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

Size pages

Other comparisons with 285/40R18

Other comparisons with 285/30R20

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