285/45R20 vs 305/45R20

305/45R20 is 0.71 in (18.0 mm) taller than 285/45R20, a +2.35% diameter difference. With the speedometer set for 285/45R20, an indicated 60 mph is 61.4 mph on 305/45R20.

Is this a safe swap?

Diameter difference +2.35%: within the ±3% guideline

By overall diameter alone, 305/45R20 falls inside the ±3% range commonly used as a guideline when replacing 285/45R20. 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.

285/45R20 vs 305/45R20 differences
Measurement	285/45R20	305/45R20	Difference	% diff
Overall diameter	30.10 in (764.5 mm)	30.81 in (782.5 mm)	+0.71 in (+18.0 mm)	+2.35%
Sidewall height	5.05 in (128.3 mm)	5.40 in (137.3 mm)	+0.35 in (+9.0 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	94.56 in (2,401.7 mm)	96.78 in (2,458.3 mm)	+2.23 in (+56.5 mm)	+2.35%
Revolutions per mile	670	655	−15	−2.30%

True speed with the speedometer calibrated for 285/45R20
Indicated	Actual on 285/45R20	Actual on 305/45R20	Difference
30 mph	30.0 mph	30.7 mph	+0.7 mph
60 mph	60.0 mph	61.4 mph	+1.4 mph
75 mph	75.0 mph	76.8 mph	+1.8 mph

Ride-height change: 305/45R20 raises the vehicle by 0.35 in (9.0 mm) at the axle (half the +18.0 mm diameter difference).

305/45R20 is 0.71 in taller than 285/45R20 and 20 mm wider, a 2.35% difference in overall diameter.

With the speedometer calibrated for 285/45R20, an indicated 60 mph is actually 61.4 mph on 305/45R20, and revolutions per mile change from 670 to 655.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 285/45R20

Other comparisons with 305/45R20

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