235/50R19 vs 255/40R20

255/40R20 is 0.22 in (5.6 mm) shorter than 235/50R19, a −0.78% diameter difference. With the speedometer set for 235/50R19, an indicated 60 mph is 59.5 mph on 255/40R20.

Is this a safe swap?

Diameter difference −0.78%: under 1%

By overall diameter alone, 255/40R20 is a near match for 235/50R19; 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.

235/50R19 vs 255/40R20 differences
Measurement	235/50R19	255/40R20	Difference	% diff
Overall diameter	28.25 in (717.6 mm)	28.03 in (712.0 mm)	−0.22 in (−5.6 mm)	−0.78%
Sidewall height	4.63 in (117.5 mm)	4.02 in (102.0 mm)	−0.61 in (−15.5 mm)	−13.19%
Section width	9.25 in (235.0 mm)	10.04 in (255.0 mm)	+0.79 in (+20.0 mm)	+8.51%
Circumference	88.76 in (2,254.4 mm)	88.06 in (2,236.8 mm)	−0.69 in (−17.6 mm)	−0.78%
Revolutions per mile	714	719	+6	+0.79%

True speed with the speedometer calibrated for 235/50R19
Indicated	Actual on 235/50R19	Actual on 255/40R20	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.5 mph	−0.5 mph
75 mph	75.0 mph	74.4 mph	−0.6 mph

Ride-height change: 255/40R20 lowers the vehicle by 0.11 in (2.8 mm) at the axle (half the −5.6 mm diameter difference).

255/40R20 is 0.22 in shorter than 235/50R19 and 20 mm wider, a 0.78% difference in overall diameter.

With the speedometer calibrated for 235/50R19, an indicated 60 mph is actually 59.5 mph on 255/40R20, and revolutions per mile change from 714 to 719.

255/40R20 mounts on a 20-inch wheel, a plus-1 fitment from the 19-inch wheel of 235/50R19.

Size pages

Other comparisons with 235/50R19

Other comparisons with 255/40R20

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