245/40R20 vs 295/30R21

295/30R21 is 0.25 in (6.4 mm) taller than 245/40R20, a +0.91% diameter difference. With the speedometer set for 245/40R20, an indicated 60 mph is 60.5 mph on 295/30R21.

Is this a safe swap?

Diameter difference +0.91%: under 1%

By overall diameter alone, 295/30R21 is a near match for 245/40R20; 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.

245/40R20 vs 295/30R21 differences
Measurement	245/40R20	295/30R21	Difference	% diff
Overall diameter	27.72 in (704.0 mm)	27.97 in (710.4 mm)	+0.25 in (+6.4 mm)	+0.91%
Sidewall height	3.86 in (98.0 mm)	3.48 in (88.5 mm)	−0.37 in (−9.5 mm)	−9.69%
Section width	9.65 in (245.0 mm)	11.61 in (295.0 mm)	+1.97 in (+50.0 mm)	+20.41%
Circumference	87.07 in (2,211.7 mm)	87.87 in (2,231.8 mm)	+0.79 in (+20.1 mm)	+0.91%
Revolutions per mile	728	721	−7	−0.90%

True speed with the speedometer calibrated for 245/40R20
Indicated	Actual on 245/40R20	Actual on 295/30R21	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.7 mph	+0.7 mph

Ride-height change: 295/30R21 raises the vehicle by 0.13 in (3.2 mm) at the axle (half the +6.4 mm diameter difference).

295/30R21 is 0.25 in taller than 245/40R20 and 50 mm wider, a 0.91% difference in overall diameter.

With the speedometer calibrated for 245/40R20, an indicated 60 mph is actually 60.5 mph on 295/30R21, and revolutions per mile change from 728 to 721.

295/30R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 245/40R20.

Size pages

Other comparisons with 245/40R20

Other comparisons with 295/30R21

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