285/45R18 vs 265/40R20

265/40R20 is 0.25 in (6.3 mm) taller than 285/45R18, a +0.88% diameter difference. With the speedometer set for 285/45R18, an indicated 60 mph is 60.5 mph on 265/40R20.

Is this a safe swap?

Diameter difference +0.88%: under 1%

By overall diameter alone, 265/40R20 is a near match for 285/45R18; 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.

285/45R18 vs 265/40R20 differences
Measurement	285/45R18	265/40R20	Difference	% diff
Overall diameter	28.10 in (713.7 mm)	28.35 in (720.0 mm)	+0.25 in (+6.3 mm)	+0.88%
Sidewall height	5.05 in (128.3 mm)	4.17 in (106.0 mm)	−0.88 in (−22.2 mm)	−17.35%
Section width	11.22 in (285.0 mm)	10.43 in (265.0 mm)	−0.79 in (−20.0 mm)	−7.02%
Circumference	88.27 in (2,242.2 mm)	89.05 in (2,261.9 mm)	+0.78 in (+19.8 mm)	+0.88%
Revolutions per mile	718	711	−6	−0.87%

True speed with the speedometer calibrated for 285/45R18
Indicated	Actual on 285/45R18	Actual on 265/40R20	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.7 mph	+0.7 mph

Ride-height change: 265/40R20 raises the vehicle by 0.12 in (3.1 mm) at the axle (half the +6.3 mm diameter difference).

265/40R20 is 0.25 in taller than 285/45R18 and 20 mm narrower, a 0.88% difference in overall diameter.

With the speedometer calibrated for 285/45R18, an indicated 60 mph is actually 60.5 mph on 265/40R20, and revolutions per mile change from 718 to 711.

265/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 285/45R18.

Size pages

Other comparisons with 285/45R18

Other comparisons with 265/40R20

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