Fuel consumption is determined by engine efficiency at the RPM and throttle required to generate enough power to overcome air drag and other forces (internal friction, rolling friction, accelerating your fat ass, gravity if you're on a hill, etc).
Here's the bad news: the power required to overcome air drag is a *cubic* function of velocity (since power is force X velocity).
I dunno of any fuel consumption graphs for bikes, but for reference here's one for a TDI (from this fairly
interesting thread):
As far decreasing speed limits goes, don't worry about it. Any efforts to do so have been crushed. Likewise, government controls (via GPS tracking, etc) would never work (due to technical and logistical difficulties as well as the masses never accepting it... we're way too libertarian in this country to live with something like that).
Keeping your speed low, engine tuned properly, tires inflated, RPMs low, and staying off the throttle will all contribute to increasing MPG (if you're really that worried about it).