[MeteorGray]

Bobo, are you using Julian's idea of using a "throttle stop" to determine the comparative aerodynamic effects?

It seems doing so would show a lot within a few miles of testing.

Edit: I just noticed your attachment video and found that you are not using a throttle stop. If I were you, I'd try using one in order to see what the changes being made do to the speed of the car on a few miles of the same stretch of road, with-and-without the bumper / undertray or whatever you're trying to test. This would show if your speed changes and how much it changes, which has implications on the aerodynamic effects and, of course, fuel mileage.