245/55R18 vs 275/40R20

275/40R20 is 0.05 in (1.3 mm) taller than 245/55R18, a +0.18% diameter difference. With the speedometer set for 245/55R18, an indicated 60 mph is 60.1 mph on 275/40R20.

Is this a safe swap?

Diameter difference +0.18%: under 1%

By overall diameter alone, 275/40R20 is a near match for 245/55R18; 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.

245/55R18 vs 275/40R20 differences
Measurement	245/55R18	275/40R20	Difference	% diff
Overall diameter	28.61 in (726.7 mm)	28.66 in (728.0 mm)	+0.05 in (+1.3 mm)	+0.18%
Sidewall height	5.31 in (134.8 mm)	4.33 in (110.0 mm)	−0.97 in (−24.7 mm)	−18.37%
Section width	9.65 in (245.0 mm)	10.83 in (275.0 mm)	+1.18 in (+30.0 mm)	+12.24%
Circumference	89.88 in (2,283.0 mm)	90.04 in (2,287.1 mm)	+0.16 in (+4.1 mm)	+0.18%
Revolutions per mile	705	704	−1	−0.18%

True speed with the speedometer calibrated for 245/55R18
Indicated	Actual on 245/55R18	Actual on 275/40R20	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.1 mph	+0.1 mph

Ride-height change: 275/40R20 raises the vehicle by 0.03 in (0.6 mm) at the axle (half the +1.3 mm diameter difference).

275/40R20 is 0.05 in taller than 245/55R18 and 30 mm wider, a 0.18% difference in overall diameter.

With the speedometer calibrated for 245/55R18, an indicated 60 mph is actually 60.1 mph on 275/40R20, and revolutions per mile change from 705 to 704.

275/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 245/55R18.

Size pages

Other comparisons with 245/55R18

Other comparisons with 275/40R20

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