I've always heard that you lose 1/2 MPG for every 5 MPH you increase your speed.
The other day with an MT trailer I determined my MPG @ my nominal travel speed of 60 MPH, and found it to be to be 8.9. I then increased my speed to 65 MPH, and sure enough, it dropped to 8.3 MPG.
Unless you actually are in a hurry to get where you are going, you are foolish to drive more than 60. So what if it takes you 10.8 hours to get where you are going instead of 10?