Guys:
I have a '96 Town Car, and used to drive up to North Carolina from Atlanta at least twice a week.
I got better gas mileage at ~72 MPH than I did at 65.
That said, at 65, I was going slower than almost everybody and noone stayed around long.
By going with the normal flow of traffic, and keeping one or two cars between me and a semi, I could raise my MPG to 27. (The cars help to elongate that air pocket that is so beneficial in drafting)
On my way to DNI , I got 24.3 mpg, and that was running an average of 67 mph.