Here is a simple idea to test?
Reverse airfoil vs almost blunt:
"I had a Plymouth Reliant Station Wagon, with a 2.4 L engine and an automatic transmission. We used it to go camping quite a lot, so I built a car top for it, the length of the top and about 20" tall. I built the front to match the angle of the windshield and just squared off the back.
In terms of packing and hauling it worked really well. I had to put air shocks on the car to get it back to level, but after that it worked great.
The only problem was that with the heavy load of people and equipment filling the entire car and car top, I would get up to about 56-58mph and then it would be like a hit a wall. If I had a really long run, I might have been able to get it up to 65mph.
One year, on a annual trip to Death Valley, in December, I wanted to take some fire wood with us, since you can't get any their. However, we were out of room, so I put a tarp on the top of the car top, stacked a pile of wood about 10" high and about 3 ' long, at the back end of the car top.
In previous years I would get about 17-18mpg, with the car top on. My gas mileage went up to about 22mpg. Additionally, when I got up to about 57mph, I didn't seem to hit the wall, like I had previously.
So I got some cardboard and fabricated a shape to put on the top of the car-top/carrier. The shape was a reverse airfoil, upper half, placed across the top of the car-top/carrier. With this shape, I was able to drive at a higher speed, I could pass other car's at 65 and so forth. I also was able to get 22mpg-25mpg.
I gave the car and car top to my parents. They made a trip with it, with 5 full size adult's and with the car packed with everything from the kitchen sink to who know's what. I asked my dad how he thought they had done on mileage and he said he had only checked the first tank of gas, which didn't make any sense, because he was always checking mileage, on trips. He said that on the first tank they got 25 mpg and he was dumbfounded. He said he didn't think anyone could ever get that kind of mileage, with that kind of load, in a car with that size of engine. So'o he quit checking it because he didn't want to try to figure out how it could be getting that, in case it was because of some etheral experience or cause he might jinx or upset the balance of, if he tried to figure it out, to closely.
I believe what was happening is that the huge box shape on the back side was basically creating a huge vacuum behind the car and that the reverse airfoil shape caused the air to be accelerated from the top of the car back down behind the car, breaking up or releasing the vacuum effect's on the car. In either case, their was definitely a force beyond just frontal surface area, which was really dragging the car down, as the speed's went up."
+ 22 to 39 % mpg gain
This wagon has a sloped back end to it...around 70* rather than 90*?
This involves increasing frontal drag substantially but still seeing substantial reduced drag overall.
I'm actually trying to test this idea with a small wagon by using some leaf filled bags, a section of waferbaord and a tarp over it all to try to duplicate this shape partially. Looks like I'm going on a trip.
a new policy....I intend to ignore the nescient...a waste of time and energy. 
Linear Mode
