I've had a few older motorbikes over the years, though never pushed to get high mpg's. My first was a 1982 Honda C70 Passport, aftermarket carb and a few other mods. I think I hit about 120mpg once, but it's not a big bike by any stretch.
I had a 1984 Toyota Tercel 4WD wagon that would hit as high as 42mpg in Missouri winters in the mid-90's, but that's as good as it gets for my cages.
My current is a 2007 Ninja 250 with about 27k miles. I've put a tall custom Lexan windscreen on for comfort, a rack with a top case and saddlebags. I've swapped the gears for a 15/41 set from JTSprockets, the tires for Kenda's K657 130/90-16 rear and K671 100/90-16 front. Lots of other little farkles, but it's mostly set up for comfortable commuting. Even so, I hit 81mpg on a fairly straight and somewhat hilly stretch of Hwy 29 North of Austin. I was running about 65mph. Don't remember the temp, but it was spring. I need to strip it down and see what I can max it out at someday...