Back when fuel wasn't real expensive (almost 15 years ago), I was running coast to coast (literally), taking ice cream of one kind from the east coast to the west coast, and taking another kind form the west coast to the east coast.
The difference for me between running 75 (as much as possilbe), and running 65, was about 1 mile per gallon. Basically about 80 less gallons of fuel difference.
Going at a modest diesel price today of $4.50, that is $360 in savings.
And contrary to what some will say, it doesn't take that much longer to get to your destination when you run slower like this. Besides, driving fast and getting there quicker just means you spend more hours at a truck stop or on the side of the street waiting to deliver
Now, if you want to factor in the fuel surcharge (assuming you are getting it), this multiplies a little more because the surcharge is based on 6mpg. If you can conserve fuel to the point you are getting over 6mpg, you actually begin making an increased profit off of the surcharge.