245/65R18 vs 305/45R20

305/45R20 is 0.27 in (6.8 mm) taller than 245/65R18, a +0.88% diameter difference. With the speedometer set for 245/65R18, an indicated 60 mph is 60.5 mph on 305/45R20.

Is this a safe swap?

Diameter difference +0.88%: under 1%

By overall diameter alone, 305/45R20 is a near match for 245/65R18; 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.

245/65R18 vs 305/45R20 differences
Measurement	245/65R18	305/45R20	Difference	% diff
Overall diameter	30.54 in (775.7 mm)	30.81 in (782.5 mm)	+0.27 in (+6.8 mm)	+0.88%
Sidewall height	6.27 in (159.3 mm)	5.40 in (137.3 mm)	−0.87 in (−22.0 mm)	−13.81%
Section width	9.65 in (245.0 mm)	12.01 in (305.0 mm)	+2.36 in (+60.0 mm)	+24.49%
Circumference	95.94 in (2,436.9 mm)	96.78 in (2,458.3 mm)	+0.84 in (+21.4 mm)	+0.88%
Revolutions per mile	660	655	−6	−0.87%

True speed with the speedometer calibrated for 245/65R18
Indicated	Actual on 245/65R18	Actual on 305/45R20	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.7 mph	+0.7 mph

Ride-height change: 305/45R20 raises the vehicle by 0.13 in (3.4 mm) at the axle (half the +6.8 mm diameter difference).

305/45R20 is 0.27 in taller than 245/65R18 and 60 mm wider, a 0.88% difference in overall diameter.

With the speedometer calibrated for 245/65R18, an indicated 60 mph is actually 60.5 mph on 305/45R20, and revolutions per mile change from 660 to 655.

305/45R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 245/65R18.

Size pages

Other comparisons with 245/65R18

Other comparisons with 305/45R20

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