245/50R20 vs 275/40R21

275/40R21 is 0.02 in (0.4 mm) taller than 245/50R20, a +0.05% diameter difference. With the speedometer set for 245/50R20, an indicated 60 mph is 60.0 mph on 275/40R21.

Is this a safe swap?

Diameter difference +0.05%: under 1%

By overall diameter alone, 275/40R21 is a near match for 245/50R20; 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.

245/50R20 vs 275/40R21 differences
Measurement	245/50R20	275/40R21	Difference	% diff
Overall diameter	29.65 in (753.0 mm)	29.66 in (753.4 mm)	+0.02 in (+0.4 mm)	+0.05%
Sidewall height	4.82 in (122.5 mm)	4.33 in (110.0 mm)	−0.49 in (−12.5 mm)	−10.20%
Section width	9.65 in (245.0 mm)	10.83 in (275.0 mm)	+1.18 in (+30.0 mm)	+12.24%
Circumference	93.13 in (2,365.6 mm)	93.18 in (2,366.9 mm)	+0.05 in (+1.3 mm)	+0.05%
Revolutions per mile	680	680	0	−0.05%

True speed with the speedometer calibrated for 245/50R20
Indicated	Actual on 245/50R20	Actual on 275/40R21
30 mph	30.0 mph	30.0 mph
60 mph	60.0 mph	60.0 mph
75 mph	75.0 mph	75.0 mph

Ride-height change: 275/40R21 raises the vehicle by 0.01 in (0.2 mm) at the axle (half the +0.4 mm diameter difference).

275/40R21 is 0.02 in taller than 245/50R20 and 30 mm wider, a 0.05% difference in overall diameter.

With the speedometer calibrated for 245/50R20, an indicated 60 mph is actually 60.0 mph on 275/40R21, and revolutions per mile change from 680 to 680.

275/40R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 245/50R20.

Size pages

Other comparisons with 245/50R20

Other comparisons with 275/40R21

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