285/55R20 vs 305/55R20

305/55R20 is 0.87 in (22.0 mm) taller than 285/55R20, a +2.68% diameter difference. With the speedometer set for 285/55R20, an indicated 60 mph is 61.6 mph on 305/55R20.

Is this a safe swap?

Diameter difference +2.68%: within the ±3% guideline

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

285/55R20 vs 305/55R20 differences
Measurement	285/55R20	305/55R20	Difference	% diff
Overall diameter	32.34 in (821.5 mm)	33.21 in (843.5 mm)	+0.87 in (+22.0 mm)	+2.68%
Sidewall height	6.17 in (156.8 mm)	6.60 in (167.8 mm)	+0.43 in (+11.0 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	101.61 in (2,580.8 mm)	104.33 in (2,649.9 mm)	+2.72 in (+69.1 mm)	+2.68%
Revolutions per mile	624	607	−16	−2.61%

True speed with the speedometer calibrated for 285/55R20
Indicated	Actual on 285/55R20	Actual on 305/55R20	Difference
30 mph	30.0 mph	30.8 mph	+0.8 mph
60 mph	60.0 mph	61.6 mph	+1.6 mph
75 mph	75.0 mph	77.0 mph	+2.0 mph

Ride-height change: 305/55R20 raises the vehicle by 0.43 in (11.0 mm) at the axle (half the +22.0 mm diameter difference).

305/55R20 is 0.87 in taller than 285/55R20 and 20 mm wider, a 2.68% difference in overall diameter.

With the speedometer calibrated for 285/55R20, an indicated 60 mph is actually 61.6 mph on 305/55R20, and revolutions per mile change from 624 to 607.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 285/55R20

Other comparisons with 305/55R20

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