295/35R20 vs 295/30R21

295/30R21 is 0.16 in (4.1 mm) shorter than 295/35R20, a −0.57% diameter difference. With the speedometer set for 295/35R20, an indicated 60 mph is 59.7 mph on 295/30R21.

Is this a safe swap?

Diameter difference −0.57%: under 1%

By overall diameter alone, 295/30R21 is a near match for 295/35R20; 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.

295/35R20 vs 295/30R21 differences
Measurement	295/35R20	295/30R21	Difference	% diff
Overall diameter	28.13 in (714.5 mm)	27.97 in (710.4 mm)	−0.16 in (−4.1 mm)	−0.57%
Sidewall height	4.06 in (103.3 mm)	3.48 in (88.5 mm)	−0.58 in (−14.7 mm)	−14.29%
Section width	11.61 in (295.0 mm)	11.61 in (295.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	88.37 in (2,244.7 mm)	87.87 in (2,231.8 mm)	−0.51 in (−12.9 mm)	−0.57%
Revolutions per mile	717	721	+4	+0.58%

True speed with the speedometer calibrated for 295/35R20
Indicated	Actual on 295/35R20	Actual on 295/30R21	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.7 mph	−0.3 mph
75 mph	75.0 mph	74.6 mph	−0.4 mph

Ride-height change: 295/30R21 lowers the vehicle by 0.08 in (2.1 mm) at the axle (half the −4.1 mm diameter difference).

295/30R21 is 0.16 in shorter than 295/35R20 and the same width, a 0.57% difference in overall diameter.

With the speedometer calibrated for 295/35R20, an indicated 60 mph is actually 59.7 mph on 295/30R21, and revolutions per mile change from 717 to 721.

295/30R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 295/35R20.

Size pages

Other comparisons with 295/35R20

Other comparisons with 295/30R21

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