235/55R18 vs 235/45R20

235/45R20 is 0.15 in (3.8 mm) taller than 235/55R18, a +0.53% diameter difference. With the speedometer set for 235/55R18, an indicated 60 mph is 60.3 mph on 235/45R20.

Is this a safe swap?

Diameter difference +0.53%: under 1%

By overall diameter alone, 235/45R20 is a near match for 235/55R18; 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.

235/55R18 vs 235/45R20 differences
Measurement	235/55R18	235/45R20	Difference	% diff
Overall diameter	28.18 in (715.7 mm)	28.33 in (719.5 mm)	+0.15 in (+3.8 mm)	+0.53%
Sidewall height	5.09 in (129.3 mm)	4.16 in (105.8 mm)	−0.93 in (−23.5 mm)	−18.18%
Section width	9.25 in (235.0 mm)	9.25 in (235.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	88.52 in (2,248.4 mm)	88.99 in (2,260.4 mm)	+0.47 in (+11.9 mm)	+0.53%
Revolutions per mile	716	712	−4	−0.53%

True speed with the speedometer calibrated for 235/55R18
Indicated	Actual on 235/55R18	Actual on 235/45R20	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.4 mph	+0.4 mph

Ride-height change: 235/45R20 raises the vehicle by 0.07 in (1.9 mm) at the axle (half the +3.8 mm diameter difference).

235/45R20 is 0.15 in taller than 235/55R18 and the same width, a 0.53% difference in overall diameter.

With the speedometer calibrated for 235/55R18, an indicated 60 mph is actually 60.3 mph on 235/45R20, and revolutions per mile change from 716 to 712.

235/45R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 235/55R18.

Size pages

Other comparisons with 235/55R18

Other comparisons with 235/45R20

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