285/50R20 vs 305/50R20

305/50R20 is 0.79 in (20.0 mm) taller than 285/50R20, a +2.52% diameter difference. With the speedometer set for 285/50R20, an indicated 60 mph is 61.5 mph on 305/50R20.

Is this a safe swap?

Diameter difference +2.52%: within the ±3% guideline

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

285/50R20 vs 305/50R20 differences
Measurement	285/50R20	305/50R20	Difference	% diff
Overall diameter	31.22 in (793.0 mm)	32.01 in (813.0 mm)	+0.79 in (+20.0 mm)	+2.52%
Sidewall height	5.61 in (142.5 mm)	6.00 in (152.5 mm)	+0.39 in (+10.0 mm)	+7.02%
Section width	11.22 in (285.0 mm)	12.01 in (305.0 mm)	+0.79 in (+20.0 mm)	+7.02%
Circumference	98.08 in (2,491.3 mm)	100.56 in (2,554.1 mm)	+2.47 in (+62.8 mm)	+2.52%
Revolutions per mile	646	630	−16	−2.46%

True speed with the speedometer calibrated for 285/50R20
Indicated	Actual on 285/50R20	Actual on 305/50R20	Difference
30 mph	30.0 mph	30.8 mph	+0.8 mph
60 mph	60.0 mph	61.5 mph	+1.5 mph
75 mph	75.0 mph	76.9 mph	+1.9 mph

Ride-height change: 305/50R20 raises the vehicle by 0.39 in (10.0 mm) at the axle (half the +20.0 mm diameter difference).

305/50R20 is 0.79 in taller than 285/50R20 and 20 mm wider, a 2.52% difference in overall diameter.

With the speedometer calibrated for 285/50R20, an indicated 60 mph is actually 61.5 mph on 305/50R20, and revolutions per mile change from 646 to 630.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 285/50R20

Other comparisons with 305/50R20

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