cooling question

"A four cylinder engine requires 1600 cfm; a six cylinder engine requires 2,000 cfm, a small V-8 engine requires 3,000 cfm and large V-8 engines require 4500+ cfm."

That's the same rule I went by when ordering. In your case (416 cubic inch) 4500+ might be the class you need fall in. I don't understand why it runs same while going down the interstate. Any type of fan is out of the equation when you are moving that fast.