What's the absolute optimum speed for fuel mileage?

My 2012 VNL64T780 with an AT2512C direct drive and a Meritor 2.64 45k/145k lbs rear ends is specked to do 56MPH at 1200RPM for 10-12 MPG at 80000 lbs on flat land and no winds.
At 68MPH the RPM is 1500. And the fuel mileage drops to 7.0 MPG.
Bobtail the MPG is 13-15. All gallons are US, not Imperial.
Disk brakes all around. 740k miles. Retired from service.
But it's white.
Does it makes me part of The White Volvo Mafia?
Possible. You'll be the Judge.

 

No, it sounds like you've got plenty of experience with them. It's probably ME that's biased. Thanks for the info.

 

It doesn't matter the make or color of your truck. Three guys in a stretched 379 crapping through a hole in the floor, taking 30's in the fuel island, parking on the shoulder, and running over a hood is just as much in that cartel as the three guys running a white Volvo.

 

The Vic was new, quiet, comfortable, powerful and affordable.
The only thing I didn't understand at first was:
Why everyone in front of my Vic changes lanes when I get behind them?
After a dozen cars moved to the right side lane,
I dropped the sun visor to look at my face in that vanity mirror.
I saw a sun tanned face, aviator sunglasses, a strong muscular built and a middle aged male.
Edit: Short sleeved white shirt and a clip on black tie with 2 red oblique lines.
And I asked myself: Do I really look like an undercover LEO?
Maybe I did. Who knows what were they thinking.
But it was funny.

 

They were the fastest car on an assembly line at about 21-22h to make it.
Impala was the second at 23h. I loved the Vic.

 

Oh, give it a rest, Tom Cruise...

 

Hey - at least mine has a container to save the dumps!