I am in the process of installing a vacuum guage in my 53 F100. With an HO 302 and AOD tranny I am getting about 12 mpg at highway speeds. I do have a stock F100 rear with 3.92 gears, but I also have 30" tall rear tires. Anyway, as I understand, the vacuum reading at highway speed will be mid range [15 hg] and up. I am taking this info from an article by Jim Linder of Bubba's Hot Rod Shop. I believe your are familiar with him from the HAMB. Joe