235/35R20 vs 285/30R20

285/30R20 is 0.26 in (6.5 mm) taller than 235/35R20, a +0.97% diameter difference. With the speedometer set for 235/35R20, an indicated 60 mph is 60.6 mph on 285/30R20.

Is this a safe swap?

Diameter difference +0.97%: under 1%

By overall diameter alone, 285/30R20 is a near match for 235/35R20; 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.

235/35R20 vs 285/30R20 differences
Measurement	235/35R20	285/30R20	Difference	% diff
Overall diameter	26.48 in (672.5 mm)	26.73 in (679.0 mm)	+0.26 in (+6.5 mm)	+0.97%
Sidewall height	3.24 in (82.3 mm)	3.37 in (85.5 mm)	+0.13 in (+3.3 mm)	+3.95%
Section width	9.25 in (235.0 mm)	11.22 in (285.0 mm)	+1.97 in (+50.0 mm)	+21.28%
Circumference	83.18 in (2,112.7 mm)	83.98 in (2,133.1 mm)	+0.80 in (+20.4 mm)	+0.97%
Revolutions per mile	762	754	−7	−0.96%

True speed with the speedometer calibrated for 235/35R20
Indicated	Actual on 235/35R20	Actual on 285/30R20	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.6 mph	+0.6 mph
75 mph	75.0 mph	75.7 mph	+0.7 mph

Ride-height change: 285/30R20 raises the vehicle by 0.13 in (3.3 mm) at the axle (half the +6.5 mm diameter difference).

285/30R20 is 0.26 in taller than 235/35R20 and 50 mm wider, a 0.97% difference in overall diameter.

With the speedometer calibrated for 235/35R20, an indicated 60 mph is actually 60.6 mph on 285/30R20, and revolutions per mile change from 762 to 754.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 235/35R20

Other comparisons with 285/30R20

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