[lago]

If I'm in 6th, I have to twist the throttle more to stay at 60-65, which just makes it feel like I'm throwing more fuel in the engine with no real benefit (after all, I'm doing the same speed, just lower revs, but with more throttle).

Generally you will achieve higher mpg by using higher throttle position / lower rpm because it takes the same amount of energy to keep the bike at that speed, but in the higher gear the engine doesn't need to spend as much energy sucking air past the butterfly valve (the story is a little different for diesel engines). Keep in mind the throttle on a gasoline engine only adjusts how easy it is for air to get into the engine, the fuel injection maintains the proper air/fuel ratio, and at higher rpm cylinders have to fill their volume more frequently. Diesel engines always allow full air in and the throttle adjusts the amount of fuel put into the engine.

Decent article:
http://autospeed.com/cms/A_110216/article.html