265/45R20 vs 305/40R20

305/40R20 is 0.22 in (5.5 mm) taller than 265/45R20, a +0.74% diameter difference. With the speedometer set for 265/45R20, an indicated 60 mph is 60.4 mph on 305/40R20.

Is this a safe swap?

Diameter difference +0.74%: under 1%

By overall diameter alone, 305/40R20 is a near match for 265/45R20; 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.

265/45R20 vs 305/40R20 differences
Measurement	265/45R20	305/40R20	Difference	% diff
Overall diameter	29.39 in (746.5 mm)	29.61 in (752.0 mm)	+0.22 in (+5.5 mm)	+0.74%
Sidewall height	4.69 in (119.3 mm)	4.80 in (122.0 mm)	+0.11 in (+2.8 mm)	+2.31%
Section width	10.43 in (265.0 mm)	12.01 in (305.0 mm)	+1.57 in (+40.0 mm)	+15.09%
Circumference	92.33 in (2,345.2 mm)	93.01 in (2,362.5 mm)	+0.68 in (+17.3 mm)	+0.74%
Revolutions per mile	686	681	−5	−0.73%

True speed with the speedometer calibrated for 265/45R20
Indicated	Actual on 265/45R20	Actual on 305/40R20	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.4 mph	+0.4 mph
75 mph	75.0 mph	75.6 mph	+0.6 mph

Ride-height change: 305/40R20 raises the vehicle by 0.11 in (2.8 mm) at the axle (half the +5.5 mm diameter difference).

305/40R20 is 0.22 in taller than 265/45R20 and 40 mm wider, a 0.74% difference in overall diameter.

With the speedometer calibrated for 265/45R20, an indicated 60 mph is actually 60.4 mph on 305/40R20, and revolutions per mile change from 686 to 681.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 265/45R20

Other comparisons with 305/40R20

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