I have one and I can tell you at speed (over 40 mph) there is no increased air flow if anything there is a decrease. I suspect what is happening is high pressure area at the base of the windshield that forces air into the vent at speed is being blocked by the top of the vent. As the air flow speed over the car increases the flow rises above the vent. I have found there is an increase in flow up to about 30 to 35 mph then it dramaticly drops off. I don't know if your old enough to remember the Cowl induction 396 SS Chevelle from 69-70 that had a door that opened facing the windshield at the rear of the hood. It used the high pressure area at the base of the windshield like a reverse ram air.