265/50R18 vs 235/45R20

235/45R20 is 0.11 in (2.7 mm) shorter than 265/50R18, a −0.37% diameter difference. With the speedometer set for 265/50R18, an indicated 60 mph is 59.8 mph on 235/45R20.

Is this a safe swap?

Diameter difference −0.37%: under 1%

By overall diameter alone, 235/45R20 is a near match for 265/50R18; 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.

265/50R18 vs 235/45R20 differences
Measurement	265/50R18	235/45R20	Difference	% diff
Overall diameter	28.43 in (722.2 mm)	28.33 in (719.5 mm)	−0.11 in (−2.7 mm)	−0.37%
Sidewall height	5.22 in (132.5 mm)	4.16 in (105.8 mm)	−1.05 in (−26.7 mm)	−20.19%
Section width	10.43 in (265.0 mm)	9.25 in (235.0 mm)	−1.18 in (−30.0 mm)	−11.32%
Circumference	89.33 in (2,268.9 mm)	88.99 in (2,260.4 mm)	−0.33 in (−8.5 mm)	−0.37%
Revolutions per mile	709	712	+3	+0.38%

True speed with the speedometer calibrated for 265/50R18
Indicated	Actual on 265/50R18	Actual on 235/45R20	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.8 mph	−0.2 mph
75 mph	75.0 mph	74.7 mph	−0.3 mph

Ride-height change: 235/45R20 lowers the vehicle by 0.05 in (1.4 mm) at the axle (half the −2.7 mm diameter difference).

235/45R20 is 0.11 in shorter than 265/50R18 and 30 mm narrower, a 0.37% difference in overall diameter.

With the speedometer calibrated for 265/50R18, an indicated 60 mph is actually 59.8 mph on 235/45R20, and revolutions per mile change from 709 to 712.

235/45R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 265/50R18.

Size pages

Other comparisons with 265/50R18

Other comparisons with 235/45R20

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