Mallinson has a series of stories (this one about an O/O) to improve efficiency.
There are several "guru"s out there; another is Kevin Rutherford.
And don't knock 6-8 mpg. Try comparison to a pickup truck pulling heavy loads (where the most I know of being towed regularly is 15k). A big truck can handle 52k in some standard configurations. The pickup is straining to maintain above 10-mpg.