235/50R18 vs 225/40R20

225/40R20 is 0.17 in (4.2 mm) shorter than 235/50R18, a −0.61% diameter difference. With the speedometer set for 235/50R18, an indicated 60 mph is 59.6 mph on 225/40R20.

Is this a safe swap?

Diameter difference −0.61%: under 1%

By overall diameter alone, 225/40R20 is a near match for 235/50R18; 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.

235/50R18 vs 225/40R20 differences
Measurement	235/50R18	225/40R20	Difference	% diff
Overall diameter	27.25 in (692.2 mm)	27.09 in (688.0 mm)	−0.17 in (−4.2 mm)	−0.61%
Sidewall height	4.63 in (117.5 mm)	3.54 in (90.0 mm)	−1.08 in (−27.5 mm)	−23.40%
Section width	9.25 in (235.0 mm)	8.86 in (225.0 mm)	−0.39 in (−10.0 mm)	−4.26%
Circumference	85.61 in (2,174.6 mm)	85.10 in (2,161.4 mm)	−0.52 in (−13.2 mm)	−0.61%
Revolutions per mile	740	745	+5	+0.61%

True speed with the speedometer calibrated for 235/50R18
Indicated	Actual on 235/50R18	Actual on 225/40R20	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.6 mph	−0.4 mph
75 mph	75.0 mph	74.5 mph	−0.5 mph

Ride-height change: 225/40R20 lowers the vehicle by 0.08 in (2.1 mm) at the axle (half the −4.2 mm diameter difference).

225/40R20 is 0.17 in shorter than 235/50R18 and 10 mm narrower, a 0.61% difference in overall diameter.

With the speedometer calibrated for 235/50R18, an indicated 60 mph is actually 59.6 mph on 225/40R20, and revolutions per mile change from 740 to 745.

225/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 235/50R18.

Size pages

Other comparisons with 235/50R18

Other comparisons with 225/40R20

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