245/50R18 vs 245/40R20

245/40R20 is 0.07 in (1.8 mm) taller than 245/50R18, a +0.26% diameter difference. With the speedometer set for 245/50R18, an indicated 60 mph is 60.2 mph on 245/40R20.

Is this a safe swap?

Diameter difference +0.26%: under 1%

By overall diameter alone, 245/40R20 is a near match for 245/50R18; 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.

245/50R18 vs 245/40R20 differences
Measurement	245/50R18	245/40R20	Difference	% diff
Overall diameter	27.65 in (702.2 mm)	27.72 in (704.0 mm)	+0.07 in (+1.8 mm)	+0.26%
Sidewall height	4.82 in (122.5 mm)	3.86 in (98.0 mm)	−0.96 in (−24.5 mm)	−20.00%
Section width	9.65 in (245.0 mm)	9.65 in (245.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	86.85 in (2,206.0 mm)	87.07 in (2,211.7 mm)	+0.22 in (+5.7 mm)	+0.26%
Revolutions per mile	730	728	−2	−0.26%

True speed with the speedometer calibrated for 245/50R18
Indicated	Actual on 245/50R18	Actual on 245/40R20	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.2 mph	+0.2 mph
75 mph	75.0 mph	75.2 mph	+0.2 mph

Ride-height change: 245/40R20 raises the vehicle by 0.04 in (0.9 mm) at the axle (half the +1.8 mm diameter difference).

245/40R20 is 0.07 in taller than 245/50R18 and the same width, a 0.26% difference in overall diameter.

With the speedometer calibrated for 245/50R18, an indicated 60 mph is actually 60.2 mph on 245/40R20, and revolutions per mile change from 730 to 728.

245/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 245/50R18.

Size pages

Other comparisons with 245/50R18

Other comparisons with 245/40R20

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