Test results from tray

I've driven with the under body tray for a week and 1 tank of gas so far. My gas mileage was 30.8 and the speed down the "hill" was the same (around 68mph) . So there was no noticeable change plus or minus from the under body tray. I'm curious if more air is flowing out my vents now or if the air through the radiator is going out the side through the wheel well. I don't know if sealing the engine bay by closing off the wheel wells to the engine bay will have a positive or negative effect. I'll be looking into adding an under body tray to cover the rear differential and seal off the underside of the rear bumper.