I am thinking of folding the bottom half back inward to get the stagnation point a bit higher. Right now it has a huge amount of down force on the front end of the car and I am sure that is not helping the mileage. I am thinking that where I put that brace, as seen by the rivets in the middle, is probably a good place to heat and bend it back. I can then use a small lip under the radiator to direct air through it and solve the fan cycling all the time while driving. On that chart it is hard to find an exact number where to make the fold but I figure anywhere is going to be better than all the way at the bottom.
My coastdown hill I normally use is
Bing Maps - Driving Directions, Traffic and Road Conditions It shows 2.2 miles and from ~4000ft to 650ft elevation. At the top of the hill just over the peak there is a sign that I can get up to 50mph when I pass. I pull the car into neutral and use the engine kill button. I then just roll down the hill and get my peak mph right at that sharp curve near the bottom. If there is no traffic and I can take the curve at full speed then to be much more accurate I can let the car continue to coast back down to 55mph. That happens around the B marker. I have tried different things to measure the accuracy and on back to back runs I have seen a difference with just the wing windows open when coasting back down to 55. I usually watch for some reference at the bottom that I pass right when the speedometer hits 55. There are usually white reflectors every so often so those are the best to use. I thought I posted a video of doing a coast down testing on here a few years ago but I searched and can't find it.
I am sure a belly pan would be much better than the back bumper the way it is now, but this was quick and easy and slightly better than stock. If I wanted to put the time and effort into it I could put a belly pan on it but for now this car is good enough till I get some mileage numbers out of it. I will probably fix the front bumper a bit more then drive it for a few weeks and see if I can get some mileage numbers that are good or at least better than I was getting.