275/55R18 vs 275/45R20

275/45R20 is 0.17 in (4.2 mm) shorter than 275/55R18, a −0.55% diameter difference. With the speedometer set for 275/55R18, an indicated 60 mph is 59.7 mph on 275/45R20.

Is this a safe swap?

Diameter difference −0.55%: under 1%

By overall diameter alone, 275/45R20 is a near match for 275/55R18; 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.

275/55R18 vs 275/45R20 differences
Measurement	275/55R18	275/45R20	Difference	% diff
Overall diameter	29.91 in (759.7 mm)	29.74 in (755.5 mm)	−0.17 in (−4.2 mm)	−0.55%
Sidewall height	5.95 in (151.3 mm)	4.87 in (123.8 mm)	−1.08 in (−27.5 mm)	−18.18%
Section width	10.83 in (275.0 mm)	10.83 in (275.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	93.96 in (2,386.7 mm)	93.44 in (2,373.5 mm)	−0.52 in (−13.2 mm)	−0.55%
Revolutions per mile	674	678	+4	+0.56%

True speed with the speedometer calibrated for 275/55R18
Indicated	Actual on 275/55R18	Actual on 275/45R20	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.7 mph	−0.3 mph
75 mph	75.0 mph	74.6 mph	−0.4 mph

Ride-height change: 275/45R20 lowers the vehicle by 0.08 in (2.1 mm) at the axle (half the −4.2 mm diameter difference).

275/45R20 is 0.17 in shorter than 275/55R18 and the same width, a 0.55% difference in overall diameter.

With the speedometer calibrated for 275/55R18, an indicated 60 mph is actually 59.7 mph on 275/45R20, and revolutions per mile change from 674 to 678.

275/45R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 275/55R18.

Size pages

Other comparisons with 275/55R18

Other comparisons with 275/45R20

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