265/50R18 vs 245/45R20

245/45R20 is 0.25 in (6.3 mm) taller than 265/50R18, a +0.87% diameter difference. With the speedometer set for 265/50R18, an indicated 60 mph is 60.5 mph on 245/45R20.

Is this a safe swap?

Diameter difference +0.87%: under 1%

By overall diameter alone, 245/45R20 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 245/45R20 differences
Measurement	265/50R18	245/45R20	Difference	% diff
Overall diameter	28.43 in (722.2 mm)	28.68 in (728.5 mm)	+0.25 in (+6.3 mm)	+0.87%
Sidewall height	5.22 in (132.5 mm)	4.34 in (110.3 mm)	−0.88 in (−22.2 mm)	−16.79%
Section width	10.43 in (265.0 mm)	9.65 in (245.0 mm)	−0.79 in (−20.0 mm)	−7.55%
Circumference	89.33 in (2,268.9 mm)	90.10 in (2,288.7 mm)	+0.78 in (+19.8 mm)	+0.87%
Revolutions per mile	709	703	−6	−0.86%

True speed with the speedometer calibrated for 265/50R18
Indicated	Actual on 265/50R18	Actual on 245/45R20	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: 245/45R20 raises the vehicle by 0.12 in (3.1 mm) at the axle (half the +6.3 mm diameter difference).

245/45R20 is 0.25 in taller than 265/50R18 and 20 mm narrower, a 0.87% difference in overall diameter.

With the speedometer calibrated for 265/50R18, an indicated 60 mph is actually 60.5 mph on 245/45R20, and revolutions per mile change from 709 to 703.

245/45R20 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 245/45R20

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