295/30R21 vs 305/30R21

305/30R21 is 0.24 in (6.0 mm) taller than 295/30R21, a +0.84% diameter difference. With the speedometer set for 295/30R21, an indicated 60 mph is 60.5 mph on 305/30R21.

Is this a safe swap?

Diameter difference +0.84%: under 1%

By overall diameter alone, 305/30R21 is a near match for 295/30R21; 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.

295/30R21 vs 305/30R21 differences
Measurement	295/30R21	305/30R21	Difference	% diff
Overall diameter	27.97 in (710.4 mm)	28.20 in (716.4 mm)	+0.24 in (+6.0 mm)	+0.84%
Sidewall height	3.48 in (88.5 mm)	3.60 in (91.5 mm)	+0.12 in (+3.0 mm)	+3.39%
Section width	11.61 in (295.0 mm)	12.01 in (305.0 mm)	+0.39 in (+10.0 mm)	+3.39%
Circumference	87.87 in (2,231.8 mm)	88.61 in (2,250.6 mm)	+0.74 in (+18.8 mm)	+0.84%
Revolutions per mile	721	715	−6	−0.84%

True speed with the speedometer calibrated for 295/30R21
Indicated	Actual on 295/30R21	Actual on 305/30R21	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	75.6 mph	+0.6 mph

Ride-height change: 305/30R21 raises the vehicle by 0.12 in (3.0 mm) at the axle (half the +6.0 mm diameter difference).

305/30R21 is 0.24 in taller than 295/30R21 and 10 mm wider, a 0.84% difference in overall diameter.

With the speedometer calibrated for 295/30R21, an indicated 60 mph is actually 60.5 mph on 305/30R21, and revolutions per mile change from 721 to 715.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 295/30R21

Other comparisons with 305/30R21

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