245/45R20 vs 245/40R21

245/40R21 is 0.04 in (0.9 mm) taller than 245/45R20, a +0.12% diameter difference. With the speedometer set for 245/45R20, an indicated 60 mph is 60.1 mph on 245/40R21.

Is this a safe swap?

Diameter difference +0.12%: under 1%

By overall diameter alone, 245/40R21 is a near match for 245/45R20; 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.

245/45R20 vs 245/40R21 differences
Measurement	245/45R20	245/40R21	Difference	% diff
Overall diameter	28.68 in (728.5 mm)	28.72 in (729.4 mm)	+0.04 in (+0.9 mm)	+0.12%
Sidewall height	4.34 in (110.3 mm)	3.86 in (98.0 mm)	−0.48 in (−12.2 mm)	−11.11%
Section width	9.65 in (245.0 mm)	9.65 in (245.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	90.10 in (2,288.7 mm)	90.22 in (2,291.5 mm)	+0.11 in (+2.8 mm)	+0.12%
Revolutions per mile	703	702	−1	−0.12%

True speed with the speedometer calibrated for 245/45R20
Indicated	Actual on 245/45R20	Actual on 245/40R21	Difference
30 mph	30.0 mph	30.0 mph	0.0 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: 245/40R21 raises the vehicle by 0.02 in (0.4 mm) at the axle (half the +0.9 mm diameter difference).

245/40R21 is 0.04 in taller than 245/45R20 and the same width, a 0.12% difference in overall diameter.

With the speedometer calibrated for 245/45R20, an indicated 60 mph is actually 60.1 mph on 245/40R21, and revolutions per mile change from 703 to 702.

245/40R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 245/45R20.

Size pages

Other comparisons with 245/45R20

Other comparisons with 245/40R21

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