245/55R20 vs 305/45R20

305/45R20 is 0.20 in (5.0 mm) taller than 245/55R20, a +0.64% diameter difference. With the speedometer set for 245/55R20, an indicated 60 mph is 60.4 mph on 305/45R20.

Is this a safe swap?

Diameter difference +0.64%: under 1%

By overall diameter alone, 305/45R20 is a near match for 245/55R20; 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.

245/55R20 vs 305/45R20 differences
Measurement	245/55R20	305/45R20	Difference	% diff
Overall diameter	30.61 in (777.5 mm)	30.81 in (782.5 mm)	+0.20 in (+5.0 mm)	+0.64%
Sidewall height	5.31 in (134.8 mm)	5.40 in (137.3 mm)	+0.10 in (+2.5 mm)	+1.86%
Section width	9.65 in (245.0 mm)	12.01 in (305.0 mm)	+2.36 in (+60.0 mm)	+24.49%
Circumference	96.16 in (2,442.6 mm)	96.78 in (2,458.3 mm)	+0.62 in (+15.7 mm)	+0.64%
Revolutions per mile	659	655	−4	−0.64%

True speed with the speedometer calibrated for 245/55R20
Indicated	Actual on 245/55R20	Actual on 305/45R20	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	75.5 mph	+0.5 mph

Ride-height change: 305/45R20 raises the vehicle by 0.10 in (2.5 mm) at the axle (half the +5.0 mm diameter difference).

305/45R20 is 0.20 in taller than 245/55R20 and 60 mm wider, a 0.64% difference in overall diameter.

With the speedometer calibrated for 245/55R20, an indicated 60 mph is actually 60.4 mph on 305/45R20, and revolutions per mile change from 659 to 655.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 245/55R20

Other comparisons with 305/45R20

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