285/30R20 vs 255/35R20

255/35R20 is 0.30 in (7.5 mm) taller than 285/30R20, a +1.10% diameter difference. With the speedometer set for 285/30R20, an indicated 60 mph is 60.7 mph on 255/35R20.

Is this a safe swap?

Diameter difference +1.10%: within the ±3% guideline

By overall diameter alone, 255/35R20 falls inside the ±3% range commonly used as a guideline when replacing 285/30R20. 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.

285/30R20 vs 255/35R20 differences
Measurement	285/30R20	255/35R20	Difference	% diff
Overall diameter	26.73 in (679.0 mm)	27.03 in (686.5 mm)	+0.30 in (+7.5 mm)	+1.10%
Sidewall height	3.37 in (85.5 mm)	3.51 in (89.3 mm)	+0.15 in (+3.8 mm)	+4.39%
Section width	11.22 in (285.0 mm)	10.04 in (255.0 mm)	−1.18 in (−30.0 mm)	−10.53%
Circumference	83.98 in (2,133.1 mm)	84.91 in (2,156.7 mm)	+0.93 in (+23.6 mm)	+1.10%
Revolutions per mile	754	746	−8	−1.09%

True speed with the speedometer calibrated for 285/30R20
Indicated	Actual on 285/30R20	Actual on 255/35R20	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.7 mph	+0.7 mph
75 mph	75.0 mph	75.8 mph	+0.8 mph

Ride-height change: 255/35R20 raises the vehicle by 0.15 in (3.8 mm) at the axle (half the +7.5 mm diameter difference).

255/35R20 is 0.30 in taller than 285/30R20 and 30 mm narrower, a 1.10% difference in overall diameter.

With the speedometer calibrated for 285/30R20, an indicated 60 mph is actually 60.7 mph on 255/35R20, and revolutions per mile change from 754 to 746.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 285/30R20

Other comparisons with 255/35R20

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