245/55R18 vs 245/45R20

245/45R20 is 0.07 in (1.8 mm) taller than 245/55R18, a +0.25% diameter difference. With the speedometer set for 245/55R18, an indicated 60 mph is 60.1 mph on 245/45R20.

Is this a safe swap?

Diameter difference +0.25%: under 1%

By overall diameter alone, 245/45R20 is a near match for 245/55R18; 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.

245/55R18 vs 245/45R20 differences
Measurement	245/55R18	245/45R20	Difference	% diff
Overall diameter	28.61 in (726.7 mm)	28.68 in (728.5 mm)	+0.07 in (+1.8 mm)	+0.25%
Sidewall height	5.31 in (134.8 mm)	4.34 in (110.3 mm)	−0.96 in (−24.5 mm)	−18.18%
Section width	9.65 in (245.0 mm)	9.65 in (245.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	89.88 in (2,283.0 mm)	90.10 in (2,288.7 mm)	+0.22 in (+5.7 mm)	+0.25%
Revolutions per mile	705	703	−2	−0.25%

True speed with the speedometer calibrated for 245/55R18
Indicated	Actual on 245/55R18	Actual on 245/45R20	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.2 mph	+0.2 mph

Ride-height change: 245/45R20 raises the vehicle by 0.04 in (0.9 mm) at the axle (half the +1.8 mm diameter difference).

245/45R20 is 0.07 in taller than 245/55R18 and the same width, a 0.25% difference in overall diameter.

With the speedometer calibrated for 245/55R18, an indicated 60 mph is actually 60.1 mph on 245/45R20, and revolutions per mile change from 705 to 703.

245/45R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 245/55R18.

Size pages

Other comparisons with 245/55R18

Other comparisons with 245/45R20

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