Dan,
I can't answer your question with any figures right now but give me a month and I can. My cousin who bought my Cobra is building various models of these generators out of stainless steel and he is hooking them to everything from his daily driver to his farm equipment. So far he hasn't had enough time to really get any good data figures, but he did say the the two pick ups run smoother. He wants to drive them for a month at least and check his mileage and everything against what it was before to see just what if any gain he gets. And being a mechanic, he will also inspect the motors at various intervals. He is using them on everything from a stock 4 cylinder to a beefed up 350 in his Nova, and of course on the diesel motors in his farm equipment. That will take longer, but he said he should have some figures on the one pickup and car as they are his and his wife's daily drivers and get a lot of miles put on them.
Edit To Add: I am not promoting this as suggested by some of the following posts. I do know for a fact that this cousin has ran his farm equipment on a variety of different fuel mixture additives that he has made, Ie Ethenol, used cooking
oil, and has had success with it. He never got double the mileage but said that it averaged about 1/4th more with no power loss or engine problems. He is just trying this. If it works good, if not all he has lost is a little time.
Ron