I know some drag week guys run 3.5" Ultraflows and they claim they are fine on the road.
I run 3" Ultraflows with turndowns in front of the rear axle and they are pretty quiet going down the road but have very little restriction.
The only problem I've had is the drivers side likes to crack where they stamped "Dynomax" in them. Drill stopped and welded and they seem to be fine.
'63 Dodge 330 11.19 @ 121 mph Pump gas, n/a, through the mufflers on street tires with 3.54's. 3,600 lbs. 10.01 @ 133mph with a 250 shot of nitrous an a splash of race gas. 1.36 60 ft. 3,700 lbs.
