Before going through the effort and expense, I would like to know if an air deflector was worth it. This discussion reminds me of the air whistles that are supposed to help avoid hitting deer. There have been lots and lots of studies that show no effect, but plenty of people still use them and believe they work.
I have done many tens of thousands of miles driving my RV and have recorded every drop of fuel I have used. My fuel mileage has been highly variable due to road delays and conditions, hills, speed, ambient temperature, wind speed and direction and who know what other factors. I would never notice a difference of a half mpg due to a deflector. It would take a controlled experiment with an accurate fuel consumption detector.
