A lot of folks post that speeds just under 65 gives them the best mpg. I would think that the best mpg is at what ever speed keeps you in the highest gear and at the lowest rpm would give you the best mpg. It will be different for each MH based on transmission gears, rear end gears, wt, and eng performance. For me with a 4 speed trans, 15k lbs wt, and 300HP, 45 mph is when it gets into high gear. So for me 50 to 55 mph is the best mpg(Slightly higher than 45 to keep it from downshifting into 3rd). I'm old so I still use a vacuum gage to know when the transmission is ready to down shift and I know if I keep it above 10 in of Hg I get my best MPG. If I drive secondary roads I will average 9-10MPG at this speed. My average at all speeds has been 8.7 mpg over 80k miles including generator usage. Headers, FF muffler, syn oil, advancing the timing,etc did not increase mpg at all. By installing a vacuum gage and using it my mpg increased by 10%.