285/40R20 vs 305/40R20

305/40R20 is 0.63 in (16.0 mm) taller than 285/40R20, a +2.17% diameter difference. With the speedometer set for 285/40R20, an indicated 60 mph is 61.3 mph on 305/40R20.

Is this a safe swap?

Diameter difference +2.17%: within the ±3% guideline

By overall diameter alone, 305/40R20 falls inside the ±3% range commonly used as a guideline when replacing 285/40R20. 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.

285/40R20 vs 305/40R20 differences
Measurement	285/40R20	305/40R20	Difference	% diff
Overall diameter	28.98 in (736.0 mm)	29.61 in (752.0 mm)	+0.63 in (+16.0 mm)	+2.17%
Sidewall height	4.49 in (114.0 mm)	4.80 in (122.0 mm)	+0.31 in (+8.0 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	91.03 in (2,312.2 mm)	93.01 in (2,362.5 mm)	+1.98 in (+50.3 mm)	+2.17%
Revolutions per mile	696	681	−15	−2.13%

True speed with the speedometer calibrated for 285/40R20
Indicated	Actual on 285/40R20	Actual on 305/40R20	Difference
30 mph	30.0 mph	30.7 mph	+0.7 mph
60 mph	60.0 mph	61.3 mph	+1.3 mph
75 mph	75.0 mph	76.6 mph	+1.6 mph

Ride-height change: 305/40R20 raises the vehicle by 0.31 in (8.0 mm) at the axle (half the +16.0 mm diameter difference).

305/40R20 is 0.63 in taller than 285/40R20 and 20 mm wider, a 2.17% difference in overall diameter.

With the speedometer calibrated for 285/40R20, an indicated 60 mph is actually 61.3 mph on 305/40R20, and revolutions per mile change from 696 to 681.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 285/40R20

Other comparisons with 305/40R20

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