305/30R19 vs 235/40R19

235/40R19 is 0.20 in (5.0 mm) taller than 305/30R19, a +0.75% diameter difference. With the speedometer set for 305/30R19, an indicated 60 mph is 60.5 mph on 235/40R19.

Is this a safe swap?

Diameter difference +0.75%: under 1%

By overall diameter alone, 235/40R19 is a near match for 305/30R19; 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.

305/30R19 vs 235/40R19 differences
Measurement	305/30R19	235/40R19	Difference	% diff
Overall diameter	26.20 in (665.6 mm)	26.40 in (670.6 mm)	+0.20 in (+5.0 mm)	+0.75%
Sidewall height	3.60 in (91.5 mm)	3.70 in (94.0 mm)	+0.10 in (+2.5 mm)	+2.73%
Section width	12.01 in (305.0 mm)	9.25 in (235.0 mm)	−2.76 in (−70.0 mm)	−22.95%
Circumference	82.32 in (2,091.0 mm)	82.94 in (2,106.8 mm)	+0.62 in (+15.7 mm)	+0.75%
Revolutions per mile	770	764	−6	−0.75%

True speed with the speedometer calibrated for 305/30R19
Indicated	Actual on 305/30R19	Actual on 235/40R19	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.6 mph	+0.6 mph

Ride-height change: 235/40R19 raises the vehicle by 0.10 in (2.5 mm) at the axle (half the +5.0 mm diameter difference).

235/40R19 is 0.20 in taller than 305/30R19 and 70 mm narrower, a 0.75% difference in overall diameter.

With the speedometer calibrated for 305/30R19, an indicated 60 mph is actually 60.5 mph on 235/40R19, and revolutions per mile change from 770 to 764.

Both sizes mount on 19-inch wheels.

Size pages

Other comparisons with 305/30R19

Other comparisons with 235/40R19

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