305/30R21 vs 265/35R21

265/35R21 is 0.10 in (2.5 mm) taller than 305/30R21, a +0.35% diameter difference. With the speedometer set for 305/30R21, an indicated 60 mph is 60.2 mph on 265/35R21.

Is this a safe swap?

Diameter difference +0.35%: under 1%

By overall diameter alone, 265/35R21 is a near match for 305/30R21; 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.

305/30R21 vs 265/35R21 differences
Measurement	305/30R21	265/35R21	Difference	% diff
Overall diameter	28.20 in (716.4 mm)	28.30 in (718.9 mm)	+0.10 in (+2.5 mm)	+0.35%
Sidewall height	3.60 in (91.5 mm)	3.65 in (92.8 mm)	+0.05 in (+1.3 mm)	+1.37%
Section width	12.01 in (305.0 mm)	10.43 in (265.0 mm)	−1.57 in (−40.0 mm)	−13.11%
Circumference	88.61 in (2,250.6 mm)	88.92 in (2,258.5 mm)	+0.31 in (+7.9 mm)	+0.35%
Revolutions per mile	715	713	−2	−0.35%

True speed with the speedometer calibrated for 305/30R21
Indicated	Actual on 305/30R21	Actual on 265/35R21	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	75.3 mph	+0.3 mph

Ride-height change: 265/35R21 raises the vehicle by 0.05 in (1.3 mm) at the axle (half the +2.5 mm diameter difference).

265/35R21 is 0.10 in taller than 305/30R21 and 40 mm narrower, a 0.35% difference in overall diameter.

With the speedometer calibrated for 305/30R21, an indicated 60 mph is actually 60.2 mph on 265/35R21, and revolutions per mile change from 715 to 713.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 305/30R21

Other comparisons with 265/35R21

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