235/45R20 vs 305/30R21

305/30R21 is 0.12 in (3.1 mm) shorter than 235/45R20, a −0.43% diameter difference. With the speedometer set for 235/45R20, an indicated 60 mph is 59.7 mph on 305/30R21.

Is this a safe swap?

Diameter difference −0.43%: under 1%

By overall diameter alone, 305/30R21 is a near match for 235/45R20; 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.

235/45R20 vs 305/30R21 differences
Measurement	235/45R20	305/30R21	Difference	% diff
Overall diameter	28.33 in (719.5 mm)	28.20 in (716.4 mm)	−0.12 in (−3.1 mm)	−0.43%
Sidewall height	4.16 in (105.8 mm)	3.60 in (91.5 mm)	−0.56 in (−14.2 mm)	−13.48%
Section width	9.25 in (235.0 mm)	12.01 in (305.0 mm)	+2.76 in (+70.0 mm)	+29.79%
Circumference	88.99 in (2,260.4 mm)	88.61 in (2,250.6 mm)	−0.38 in (−9.7 mm)	−0.43%
Revolutions per mile	712	715	+3	+0.43%

True speed with the speedometer calibrated for 235/45R20
Indicated	Actual on 235/45R20	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	74.7 mph	−0.3 mph

Ride-height change: 305/30R21 lowers the vehicle by 0.06 in (1.6 mm) at the axle (half the −3.1 mm diameter difference).

305/30R21 is 0.12 in shorter than 235/45R20 and 70 mm wider, a 0.43% difference in overall diameter.

With the speedometer calibrated for 235/45R20, an indicated 60 mph is actually 59.7 mph on 305/30R21, and revolutions per mile change from 712 to 715.

305/30R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 235/45R20.

Size pages

Other comparisons with 235/45R20

Other comparisons with 305/30R21

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