275/60R18 vs 245/55R20

245/55R20 is 0.38 in (9.7 mm) shorter than 275/60R18, a −1.23% diameter difference. With the speedometer set for 275/60R18, an indicated 60 mph is 59.3 mph on 245/55R20.

Is this a safe swap?

Diameter difference −1.23%: within the ±3% guideline

By overall diameter alone, 245/55R20 falls inside the ±3% range commonly used as a guideline when replacing 275/60R18. 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.

275/60R18 vs 245/55R20 differences
Measurement	275/60R18	245/55R20	Difference	% diff
Overall diameter	30.99 in (787.2 mm)	30.61 in (777.5 mm)	−0.38 in (−9.7 mm)	−1.23%
Sidewall height	6.50 in (165.0 mm)	5.31 in (134.8 mm)	−1.19 in (−30.2 mm)	−18.33%
Section width	10.83 in (275.0 mm)	9.65 in (245.0 mm)	−1.18 in (−30.0 mm)	−10.91%
Circumference	97.36 in (2,473.1 mm)	96.16 in (2,442.6 mm)	−1.20 in (−30.5 mm)	−1.23%
Revolutions per mile	651	659	+8	+1.25%

True speed with the speedometer calibrated for 275/60R18
Indicated	Actual on 275/60R18	Actual on 245/55R20	Difference
30 mph	30.0 mph	29.6 mph	−0.4 mph
60 mph	60.0 mph	59.3 mph	−0.7 mph
75 mph	75.0 mph	74.1 mph	−0.9 mph

Ride-height change: 245/55R20 lowers the vehicle by 0.19 in (4.9 mm) at the axle (half the −9.7 mm diameter difference).

245/55R20 is 0.38 in shorter than 275/60R18 and 30 mm narrower, a 1.23% difference in overall diameter.

With the speedometer calibrated for 275/60R18, an indicated 60 mph is actually 59.3 mph on 245/55R20, and revolutions per mile change from 651 to 659.

245/55R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 275/60R18.

Size pages

Other comparisons with 275/60R18

Other comparisons with 245/55R20

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