265/50R18 vs 265/40R20

265/40R20 is 0.09 in (2.2 mm) shorter than 265/50R18, a −0.30% diameter difference. With the speedometer set for 265/50R18, an indicated 60 mph is 59.8 mph on 265/40R20.

Is this a safe swap?

Diameter difference −0.30%: under 1%

By overall diameter alone, 265/40R20 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 265/40R20 differences
Measurement	265/50R18	265/40R20	Difference	% diff
Overall diameter	28.43 in (722.2 mm)	28.35 in (720.0 mm)	−0.09 in (−2.2 mm)	−0.30%
Sidewall height	5.22 in (132.5 mm)	4.17 in (106.0 mm)	−1.04 in (−26.5 mm)	−20.00%
Section width	10.43 in (265.0 mm)	10.43 in (265.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	89.33 in (2,268.9 mm)	89.05 in (2,261.9 mm)	−0.27 in (−6.9 mm)	−0.30%
Revolutions per mile	709	711	+2	+0.31%

True speed with the speedometer calibrated for 265/50R18
Indicated	Actual on 265/50R18	Actual on 265/40R20	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	74.8 mph	−0.2 mph

Ride-height change: 265/40R20 lowers the vehicle by 0.04 in (1.1 mm) at the axle (half the −2.2 mm diameter difference).

265/40R20 is 0.09 in shorter than 265/50R18 and the same width, a 0.30% difference in overall diameter.

With the speedometer calibrated for 265/50R18, an indicated 60 mph is actually 59.8 mph on 265/40R20, and revolutions per mile change from 709 to 711.

265/40R20 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 265/40R20

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