285/35R19 vs 285/30R20

285/30R20 is 0.12 in (3.1 mm) shorter than 285/35R19, a −0.45% diameter difference. With the speedometer set for 285/35R19, an indicated 60 mph is 59.7 mph on 285/30R20.

Is this a safe swap?

Diameter difference −0.45%: under 1%

By overall diameter alone, 285/30R20 is a near match for 285/35R19; 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.

285/35R19 vs 285/30R20 differences
Measurement	285/35R19	285/30R20	Difference	% diff
Overall diameter	26.85 in (682.1 mm)	26.73 in (679.0 mm)	−0.12 in (−3.1 mm)	−0.45%
Sidewall height	3.93 in (99.8 mm)	3.37 in (85.5 mm)	−0.56 in (−14.2 mm)	−14.29%
Section width	11.22 in (285.0 mm)	11.22 in (285.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	84.37 in (2,142.9 mm)	83.98 in (2,133.1 mm)	−0.38 in (−9.7 mm)	−0.45%
Revolutions per mile	751	754	+3	+0.46%

True speed with the speedometer calibrated for 285/35R19
Indicated	Actual on 285/35R19	Actual on 285/30R20	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.7 mph	−0.3 mph
75 mph	75.0 mph	74.7 mph	−0.3 mph

Ride-height change: 285/30R20 lowers the vehicle by 0.06 in (1.5 mm) at the axle (half the −3.1 mm diameter difference).

285/30R20 is 0.12 in shorter than 285/35R19 and the same width, a 0.45% difference in overall diameter.

With the speedometer calibrated for 285/35R19, an indicated 60 mph is actually 59.7 mph on 285/30R20, and revolutions per mile change from 751 to 754.

285/30R20 mounts on a 20-inch wheel, a plus-1 fitment from the 19-inch wheel of 285/35R19.

Size pages

Other comparisons with 285/35R19

Other comparisons with 285/30R20

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