265/40R20 vs 305/30R21

305/30R21 is 0.14 in (3.6 mm) shorter than 265/40R20, a −0.50% diameter difference. With the speedometer set for 265/40R20, an indicated 60 mph is 59.7 mph on 305/30R21.

Is this a safe swap?

Diameter difference −0.50%: under 1%

By overall diameter alone, 305/30R21 is a near match for 265/40R20; 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.

265/40R20 vs 305/30R21 differences
Measurement	265/40R20	305/30R21	Difference	% diff
Overall diameter	28.35 in (720.0 mm)	28.20 in (716.4 mm)	−0.14 in (−3.6 mm)	−0.50%
Sidewall height	4.17 in (106.0 mm)	3.60 in (91.5 mm)	−0.57 in (−14.5 mm)	−13.68%
Section width	10.43 in (265.0 mm)	12.01 in (305.0 mm)	+1.57 in (+40.0 mm)	+15.09%
Circumference	89.05 in (2,261.9 mm)	88.61 in (2,250.6 mm)	−0.45 in (−11.3 mm)	−0.50%
Revolutions per mile	711	715	+4	+0.50%

True speed with the speedometer calibrated for 265/40R20
Indicated	Actual on 265/40R20	Actual on 305/30R21	Difference
30 mph	30.0 mph	29.9 mph	−0.1 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: 305/30R21 lowers the vehicle by 0.07 in (1.8 mm) at the axle (half the −3.6 mm diameter difference).

305/30R21 is 0.14 in shorter than 265/40R20 and 40 mm wider, a 0.50% difference in overall diameter.

With the speedometer calibrated for 265/40R20, an indicated 60 mph is actually 59.7 mph on 305/30R21, and revolutions per mile change from 711 to 715.

305/30R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 265/40R20.

Size pages

Other comparisons with 265/40R20

Other comparisons with 305/30R21

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