255/40R20 vs 305/30R21

305/30R21 is 0.17 in (4.4 mm) taller than 255/40R20, a +0.62% diameter difference. With the speedometer set for 255/40R20, an indicated 60 mph is 60.4 mph on 305/30R21.

Is this a safe swap?

Diameter difference +0.62%: under 1%

By overall diameter alone, 305/30R21 is a near match for 255/40R20; 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.

255/40R20 vs 305/30R21 differences
Measurement	255/40R20	305/30R21	Difference	% diff
Overall diameter	28.03 in (712.0 mm)	28.20 in (716.4 mm)	+0.17 in (+4.4 mm)	+0.62%
Sidewall height	4.02 in (102.0 mm)	3.60 in (91.5 mm)	−0.41 in (−10.5 mm)	−10.29%
Section width	10.04 in (255.0 mm)	12.01 in (305.0 mm)	+1.97 in (+50.0 mm)	+19.61%
Circumference	88.06 in (2,236.8 mm)	88.61 in (2,250.6 mm)	+0.54 in (+13.8 mm)	+0.62%
Revolutions per mile	719	715	−4	−0.61%

True speed with the speedometer calibrated for 255/40R20
Indicated	Actual on 255/40R20	Actual on 305/30R21	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.4 mph	+0.4 mph
75 mph	75.0 mph	75.5 mph	+0.5 mph

Ride-height change: 305/30R21 raises the vehicle by 0.09 in (2.2 mm) at the axle (half the +4.4 mm diameter difference).

305/30R21 is 0.17 in taller than 255/40R20 and 50 mm wider, a 0.62% difference in overall diameter.

With the speedometer calibrated for 255/40R20, an indicated 60 mph is actually 60.4 mph on 305/30R21, and revolutions per mile change from 719 to 715.

305/30R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 255/40R20.

Size pages

Other comparisons with 255/40R20

Other comparisons with 305/30R21

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