245/55R20 vs 275/45R21

275/45R21 is 0.13 in (3.4 mm) taller than 245/55R20, a +0.44% diameter difference. With the speedometer set for 245/55R20, an indicated 60 mph is 60.3 mph on 275/45R21.

Is this a safe swap?

Diameter difference +0.44%: under 1%

By overall diameter alone, 275/45R21 is a near match for 245/55R20; 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.

245/55R20 vs 275/45R21 differences
Measurement	245/55R20	275/45R21	Difference	% diff
Overall diameter	30.61 in (777.5 mm)	30.74 in (780.9 mm)	+0.13 in (+3.4 mm)	+0.44%
Sidewall height	5.31 in (134.8 mm)	4.87 in (123.8 mm)	−0.43 in (−11.0 mm)	−8.16%
Section width	9.65 in (245.0 mm)	10.83 in (275.0 mm)	+1.18 in (+30.0 mm)	+12.24%
Circumference	96.16 in (2,442.6 mm)	96.59 in (2,453.3 mm)	+0.42 in (+10.7 mm)	+0.44%
Revolutions per mile	659	656	−3	−0.44%

True speed with the speedometer calibrated for 245/55R20
Indicated	Actual on 245/55R20	Actual on 275/45R21	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.3 mph	+0.3 mph

Ride-height change: 275/45R21 raises the vehicle by 0.07 in (1.7 mm) at the axle (half the +3.4 mm diameter difference).

275/45R21 is 0.13 in taller than 245/55R20 and 30 mm wider, a 0.44% difference in overall diameter.

With the speedometer calibrated for 245/55R20, an indicated 60 mph is actually 60.3 mph on 275/45R21, and revolutions per mile change from 659 to 656.

275/45R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 245/55R20.

Size pages

Other comparisons with 245/55R20

Other comparisons with 275/45R21

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