285/60R18 vs 305/50R20

305/50R20 is 0.54 in (13.8 mm) taller than 285/60R18, a +1.73% diameter difference. With the speedometer set for 285/60R18, an indicated 60 mph is 61.0 mph on 305/50R20.

Is this a safe swap?

Diameter difference +1.73%: within the ±3% guideline

By overall diameter alone, 305/50R20 falls inside the ±3% range commonly used as a guideline when replacing 285/60R18. 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.

285/60R18 vs 305/50R20 differences
Measurement	285/60R18	305/50R20	Difference	% diff
Overall diameter	31.46 in (799.2 mm)	32.01 in (813.0 mm)	+0.54 in (+13.8 mm)	+1.73%
Sidewall height	6.73 in (171.0 mm)	6.00 in (152.5 mm)	−0.73 in (−18.5 mm)	−10.82%
Section width	11.22 in (285.0 mm)	12.01 in (305.0 mm)	+0.79 in (+20.0 mm)	+7.02%
Circumference	98.85 in (2,510.8 mm)	100.56 in (2,554.1 mm)	+1.71 in (+43.4 mm)	+1.73%
Revolutions per mile	641	630	−11	−1.70%

True speed with the speedometer calibrated for 285/60R18
Indicated	Actual on 285/60R18	Actual on 305/50R20	Difference
30 mph	30.0 mph	30.5 mph	+0.5 mph
60 mph	60.0 mph	61.0 mph	+1.0 mph
75 mph	75.0 mph	76.3 mph	+1.3 mph

Ride-height change: 305/50R20 raises the vehicle by 0.27 in (6.9 mm) at the axle (half the +13.8 mm diameter difference).

305/50R20 is 0.54 in taller than 285/60R18 and 20 mm wider, a 1.73% difference in overall diameter.

With the speedometer calibrated for 285/60R18, an indicated 60 mph is actually 61.0 mph on 305/50R20, and revolutions per mile change from 641 to 630.

305/50R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 285/60R18.

Size pages

Other comparisons with 285/60R18

Other comparisons with 305/50R20

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