Fashioned a boat tail out of cardboard and coloroplast. Did bi-directional runs North and South. Winds were 350@9 at the beginning and 360@14 after the end of the last run. Temperature remained steady at 45. Speed was 55. No cruise control but scan gauge showed average speed 55. Increase in FE 2.17%
Overall average 45.75 with BT
Overall average without 44.775
Total increase of 2.17%
Little disappointed with the results. Was expecting double what I ended up seeing. Although I am happy with the 45 MPG highway. I think that with cheap fabrications and crude attachment procedures that 3% is about it as far as mods go. Seems like every aero mods hit?s somewhere around 3%. Just proving that there is no silver bullet you just need to make sure your clip is full.
The new VW Polo diesel has a mini boat-tail. It's hard to see in this picture, but click the link below for a super huge zoom. You can see they added a plastic/fiberglass extension to the roofline & C pillar (it goes several inches past the back glass).