285/45R20 vs 295/40R21

295/40R21 is 0.19 in (4.9 mm) taller than 285/45R20, a +0.64% diameter difference. With the speedometer set for 285/45R20, an indicated 60 mph is 60.4 mph on 295/40R21.

Is this a safe swap?

Diameter difference +0.64%: under 1%

By overall diameter alone, 295/40R21 is a near match for 285/45R20; 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.

285/45R20 vs 295/40R21 differences
Measurement	285/45R20	295/40R21	Difference	% diff
Overall diameter	30.10 in (764.5 mm)	30.29 in (769.4 mm)	+0.19 in (+4.9 mm)	+0.64%
Sidewall height	5.05 in (128.3 mm)	4.65 in (118.0 mm)	−0.40 in (−10.2 mm)	−7.99%
Section width	11.22 in (285.0 mm)	11.61 in (295.0 mm)	+0.39 in (+10.0 mm)	+3.51%
Circumference	94.56 in (2,401.7 mm)	95.16 in (2,417.1 mm)	+0.61 in (+15.4 mm)	+0.64%
Revolutions per mile	670	666	−4	−0.64%

True speed with the speedometer calibrated for 285/45R20
Indicated	Actual on 285/45R20	Actual on 295/40R21	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.4 mph	+0.4 mph
75 mph	75.0 mph	75.5 mph	+0.5 mph

Ride-height change: 295/40R21 raises the vehicle by 0.10 in (2.4 mm) at the axle (half the +4.9 mm diameter difference).

295/40R21 is 0.19 in taller than 285/45R20 and 10 mm wider, a 0.64% difference in overall diameter.

With the speedometer calibrated for 285/45R20, an indicated 60 mph is actually 60.4 mph on 295/40R21, and revolutions per mile change from 670 to 666.

295/40R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 285/45R20.

Size pages

Other comparisons with 285/45R20

Other comparisons with 295/40R21

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