305/50R20 vs 285/55R20

285/55R20 is 0.33 in (8.5 mm) taller than 305/50R20, a +1.05% diameter difference. With the speedometer set for 305/50R20, an indicated 60 mph is 60.6 mph on 285/55R20.

Is this a safe swap?

Diameter difference +1.05%: within the ±3% guideline

By overall diameter alone, 285/55R20 falls inside the ±3% range commonly used as a guideline when replacing 305/50R20. 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.

305/50R20 vs 285/55R20 differences
Measurement	305/50R20	285/55R20	Difference	% diff
Overall diameter	32.01 in (813.0 mm)	32.34 in (821.5 mm)	+0.33 in (+8.5 mm)	+1.05%
Sidewall height	6.00 in (152.5 mm)	6.17 in (156.8 mm)	+0.17 in (+4.3 mm)	+2.79%
Section width	12.01 in (305.0 mm)	11.22 in (285.0 mm)	−0.79 in (−20.0 mm)	−6.56%
Circumference	100.56 in (2,554.1 mm)	101.61 in (2,580.8 mm)	+1.05 in (+26.7 mm)	+1.05%
Revolutions per mile	630	624	−7	−1.03%

True speed with the speedometer calibrated for 305/50R20
Indicated	Actual on 305/50R20	Actual on 285/55R20	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	75.8 mph	+0.8 mph

Ride-height change: 285/55R20 raises the vehicle by 0.17 in (4.3 mm) at the axle (half the +8.5 mm diameter difference).

285/55R20 is 0.33 in taller than 305/50R20 and 20 mm narrower, a 1.05% difference in overall diameter.

With the speedometer calibrated for 305/50R20, an indicated 60 mph is actually 60.6 mph on 285/55R20, and revolutions per mile change from 630 to 624.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 305/50R20

Other comparisons with 285/55R20

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