295/40R19 vs 305/30R21

305/30R21 is 0.09 in (2.2 mm) shorter than 295/40R19, a −0.31% diameter difference. With the speedometer set for 295/40R19, an indicated 60 mph is 59.8 mph on 305/30R21.

Is this a safe swap?

Diameter difference −0.31%: under 1%

By overall diameter alone, 305/30R21 is a near match for 295/40R19; 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.

295/40R19 vs 305/30R21 differences
Measurement	295/40R19	305/30R21	Difference	% diff
Overall diameter	28.29 in (718.6 mm)	28.20 in (716.4 mm)	−0.09 in (−2.2 mm)	−0.31%
Sidewall height	4.65 in (118.0 mm)	3.60 in (91.5 mm)	−1.04 in (−26.5 mm)	−22.46%
Section width	11.61 in (295.0 mm)	12.01 in (305.0 mm)	+0.39 in (+10.0 mm)	+3.39%
Circumference	88.88 in (2,257.5 mm)	88.61 in (2,250.6 mm)	−0.27 in (−6.9 mm)	−0.31%
Revolutions per mile	713	715	+2	+0.31%

True speed with the speedometer calibrated for 295/40R19
Indicated	Actual on 295/40R19	Actual on 305/30R21	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.8 mph	−0.2 mph
75 mph	75.0 mph	74.8 mph	−0.2 mph

Ride-height change: 305/30R21 lowers the vehicle by 0.04 in (1.1 mm) at the axle (half the −2.2 mm diameter difference).

305/30R21 is 0.09 in shorter than 295/40R19 and 10 mm wider, a 0.31% difference in overall diameter.

With the speedometer calibrated for 295/40R19, an indicated 60 mph is actually 59.8 mph on 305/30R21, and revolutions per mile change from 713 to 715.

305/30R21 mounts on a 21-inch wheel, a plus-2 fitment from the 19-inch wheel of 295/40R19.

Size pages

Other comparisons with 295/40R19

Other comparisons with 305/30R21

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