255/55R18 vs 255/45R20

255/45R20 is 0.01 in (0.2 mm) shorter than 255/55R18, a −0.03% diameter difference. With the speedometer set for 255/55R18, an indicated 60 mph is 60.0 mph on 255/45R20.

Is this a safe swap?

Diameter difference −0.03%: under 1%

By overall diameter alone, 255/45R20 is a near match for 255/55R18; 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.

255/55R18 vs 255/45R20 differences
Measurement	255/55R18	255/45R20	Difference	% diff
Overall diameter	29.04 in (737.7 mm)	29.04 in (737.5 mm)	−0.01 in (−0.2 mm)	−0.03%
Sidewall height	5.52 in (140.3 mm)	4.52 in (114.8 mm)	−1.00 in (−25.5 mm)	−18.18%
Section width	10.04 in (255.0 mm)	10.04 in (255.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	91.24 in (2,317.6 mm)	91.22 in (2,316.9 mm)	−0.02 in (−0.6 mm)	−0.03%
Revolutions per mile	694	695	0	+0.03%

True speed with the speedometer calibrated for 255/55R18
Indicated	Actual on 255/55R18	Actual on 255/45R20
30 mph	30.0 mph	30.0 mph
60 mph	60.0 mph	60.0 mph
75 mph	75.0 mph	75.0 mph

Ride-height change: 255/45R20 lowers the vehicle by 0.00 in (0.1 mm) at the axle (half the −0.2 mm diameter difference).

255/45R20 is 0.01 in shorter than 255/55R18 and the same width, a 0.03% difference in overall diameter.

With the speedometer calibrated for 255/55R18, an indicated 60 mph is actually 60.0 mph on 255/45R20, and revolutions per mile change from 694 to 695.

255/45R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/55R18.

Size pages

Other comparisons with 255/55R18

Other comparisons with 255/45R20

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