Fuel economy and additives

I have tried a number of things over the past 15 years but only found one thing that actually worked. When I first bought the MH the exhaust manifolds were leaking so I added Thorley Tri Y headers. This increased performance but did not increase MPG, added free flow muffler, CAI and advanced timing from 4 degrees BTDC base to 9 all increased performance but saw no MPG increase. Went to synthetic oil saw no performance or MPG increase. Tried acetone in the gas once and got 9.5 mpg but never followed through to see if that was an anomaly. I then found an old vacuum gauge in my garage that I installed and my MPG went from 8.5 to 8.7 fairly consistently if I kept it over 10 in of HG. I tracked every fill up over 90,000 miles of travel and noticed better MPGs in mountainous hiway roads that are fairly straight where I could give it gas when I saw a hill then slowly back of the gas by glancing at the vacuum gauge because I new when it was going to down shift. Mountainous roads with a lot of switch backs and dirt roads up like up in Alaska drop the MPG to 8.4. I see no MPG difference towing a 2400 lb car or not towing.