275/55R18 vs 245/50R20

245/50R20 is 0.26 in (6.7 mm) shorter than 275/55R18, a −0.88% diameter difference. With the speedometer set for 275/55R18, an indicated 60 mph is 59.5 mph on 245/50R20.

Is this a safe swap?

Diameter difference −0.88%: under 1%

By overall diameter alone, 245/50R20 is a near match for 275/55R18; 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.

275/55R18 vs 245/50R20 differences
Measurement	275/55R18	245/50R20	Difference	% diff
Overall diameter	29.91 in (759.7 mm)	29.65 in (753.0 mm)	−0.26 in (−6.7 mm)	−0.88%
Sidewall height	5.95 in (151.3 mm)	4.82 in (122.5 mm)	−1.13 in (−28.7 mm)	−19.01%
Section width	10.83 in (275.0 mm)	9.65 in (245.0 mm)	−1.18 in (−30.0 mm)	−10.91%
Circumference	93.96 in (2,386.7 mm)	93.13 in (2,365.6 mm)	−0.83 in (−21.0 mm)	−0.88%
Revolutions per mile	674	680	+6	+0.89%

True speed with the speedometer calibrated for 275/55R18
Indicated	Actual on 275/55R18	Actual on 245/50R20	Difference
30 mph	30.0 mph	29.7 mph	−0.3 mph
60 mph	60.0 mph	59.5 mph	−0.5 mph
75 mph	75.0 mph	74.3 mph	−0.7 mph

Ride-height change: 245/50R20 lowers the vehicle by 0.13 in (3.4 mm) at the axle (half the −6.7 mm diameter difference).

245/50R20 is 0.26 in shorter than 275/55R18 and 30 mm narrower, a 0.88% difference in overall diameter.

With the speedometer calibrated for 275/55R18, an indicated 60 mph is actually 59.5 mph on 245/50R20, and revolutions per mile change from 674 to 680.

245/50R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 275/55R18.

Size pages

Other comparisons with 275/55R18

Other comparisons with 245/50R20

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