265/60R18 vs 305/45R20

305/45R20 is 0.29 in (7.3 mm) taller than 265/60R18, a +0.94% diameter difference. With the speedometer set for 265/60R18, an indicated 60 mph is 60.6 mph on 305/45R20.

Is this a safe swap?

Diameter difference +0.94%: under 1%

By overall diameter alone, 305/45R20 is a near match for 265/60R18; the speedometer changes by less than 1%. 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.

265/60R18 vs 305/45R20 differences
Measurement	265/60R18	305/45R20	Difference	% diff
Overall diameter	30.52 in (775.2 mm)	30.81 in (782.5 mm)	+0.29 in (+7.3 mm)	+0.94%
Sidewall height	6.26 in (159.0 mm)	5.40 in (137.3 mm)	−0.86 in (−21.7 mm)	−13.68%
Section width	10.43 in (265.0 mm)	12.01 in (305.0 mm)	+1.57 in (+40.0 mm)	+15.09%
Circumference	95.88 in (2,435.4 mm)	96.78 in (2,458.3 mm)	+0.90 in (+22.9 mm)	+0.94%
Revolutions per mile	661	655	−6	−0.93%

True speed with the speedometer calibrated for 265/60R18
Indicated	Actual on 265/60R18	Actual on 305/45R20	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.6 mph	+0.6 mph
75 mph	75.0 mph	75.7 mph	+0.7 mph

Ride-height change: 305/45R20 raises the vehicle by 0.14 in (3.6 mm) at the axle (half the +7.3 mm diameter difference).

305/45R20 is 0.29 in taller than 265/60R18 and 40 mm wider, a 0.94% difference in overall diameter.

With the speedometer calibrated for 265/60R18, an indicated 60 mph is actually 60.6 mph on 305/45R20, and revolutions per mile change from 661 to 655.

305/45R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 265/60R18.

Size pages

Other comparisons with 265/60R18

Other comparisons with 305/45R20

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