295/35R20 vs 305/30R21

305/30R21 is 0.07 in (1.9 mm) taller than 295/35R20, a +0.27% diameter difference. With the speedometer set for 295/35R20, an indicated 60 mph is 60.2 mph on 305/30R21.

Is this a safe swap?

Diameter difference +0.27%: under 1%

By overall diameter alone, 305/30R21 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 305/30R21 differences
Measurement	295/35R20	305/30R21	Difference	% diff
Overall diameter	28.13 in (714.5 mm)	28.20 in (716.4 mm)	+0.07 in (+1.9 mm)	+0.27%
Sidewall height	4.06 in (103.3 mm)	3.60 in (91.5 mm)	−0.46 in (−11.7 mm)	−11.38%
Section width	11.61 in (295.0 mm)	12.01 in (305.0 mm)	+0.39 in (+10.0 mm)	+3.39%
Circumference	88.37 in (2,244.7 mm)	88.61 in (2,250.6 mm)	+0.24 in (+6.0 mm)	+0.27%
Revolutions per mile	717	715	−2	−0.27%

True speed with the speedometer calibrated for 295/35R20
Indicated	Actual on 295/35R20	Actual on 305/30R21	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.2 mph	+0.2 mph
75 mph	75.0 mph	75.2 mph	+0.2 mph

Ride-height change: 305/30R21 raises the vehicle by 0.04 in (0.9 mm) at the axle (half the +1.9 mm diameter difference).

305/30R21 is 0.07 in taller than 295/35R20 and 10 mm wider, a 0.27% difference in overall diameter.

With the speedometer calibrated for 295/35R20, an indicated 60 mph is actually 60.2 mph on 305/30R21, and revolutions per mile change from 717 to 715.

305/30R21 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 305/30R21

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