245/60R18 vs 275/45R20

275/45R20 is 0.17 in (4.3 mm) taller than 245/60R18, a +0.57% diameter difference. With the speedometer set for 245/60R18, an indicated 60 mph is 60.3 mph on 275/45R20.

Is this a safe swap?

Diameter difference +0.57%: under 1%

By overall diameter alone, 275/45R20 is a near match for 245/60R18; 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.

245/60R18 vs 275/45R20 differences
Measurement	245/60R18	275/45R20	Difference	% diff
Overall diameter	29.57 in (751.2 mm)	29.74 in (755.5 mm)	+0.17 in (+4.3 mm)	+0.57%
Sidewall height	5.79 in (147.0 mm)	4.87 in (123.8 mm)	−0.92 in (−23.2 mm)	−15.82%
Section width	9.65 in (245.0 mm)	10.83 in (275.0 mm)	+1.18 in (+30.0 mm)	+12.24%
Circumference	92.91 in (2,360.0 mm)	93.44 in (2,373.5 mm)	+0.53 in (+13.5 mm)	+0.57%
Revolutions per mile	682	678	−4	−0.57%

True speed with the speedometer calibrated for 245/60R18
Indicated	Actual on 245/60R18	Actual on 275/45R20	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.4 mph	+0.4 mph

Ride-height change: 275/45R20 raises the vehicle by 0.08 in (2.1 mm) at the axle (half the +4.3 mm diameter difference).

275/45R20 is 0.17 in taller than 245/60R18 and 30 mm wider, a 0.57% difference in overall diameter.

With the speedometer calibrated for 245/60R18, an indicated 60 mph is actually 60.3 mph on 275/45R20, and revolutions per mile change from 682 to 678.

275/45R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 245/60R18.

Size pages

Other comparisons with 245/60R18

Other comparisons with 275/45R20

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