What's the absolute optimum speed for fuel mileage?

An experienced driver can out perform cruise control. If you’re not conscientious about the surroundings, then maybe the cruise is what you should use.
I only, and I mean only, use cruise control when I am on flat open ground.

 

Every truck spec will have a different outcome as to speed
Also
That $50 savings works when you have nowhere you need to be but that same $50 can cost you thousands if say you deliver Monday instead of Friday AM and are able to run the weekend

 

Not to mention it's really not even a full $50 in savings. You're coughing up 80 miles worth of pay/productivity.

I run 65mph at night, if I'm not pushing to be somewhere. I don't do it for the fuel mileage. I do it because it's easier on the truck and tires. There could be an argument you're saving money right there, but honestly it's just more of a "feeling" I get, than something I could quantify on paper. Plus it's less stressful when I need to pass someone, knowing I can grab a quick 8mph and get around them.

Then again, my truck doesn't break 6mpg unless it's empty. Diesel fuel is something I look at as a necessary evil. Costs money to make money, and I'm not one to worry over every dime. Life's too short to not let that turbo whistle.

 

The fuel you burn is lost in 3 ways:
Internal friction. 33% Engine, transmission, rear ends. You minimize it, you win.
Rolling resistance. 33?% The road and tire surface/inflation determines how much is it.
Wind resistance. 33%. The speed you roll determines it. At 70 km/h is almost none.
Google or DuckDuck all three subjects if you are interested. You'll learn a lot.
In Europe the truck speed was regulated at 80km/h for years. They'll increase to 100km/h soon.

 

It is lifestyle. If I have a load 1,950 miles away, I can push for three days, or mosey for four days. But if I mosey, I just gave up $1,500, because I just lost a day.

Right there. If you pushed at the speed limit, you could grab another load. You're only on this Earth so many days.

Driving slow is relaxing. I know exactly where you're coming from. But at 62mph, you're a rolling road block, and everyone is stuck grabbing the hammer to get around you. It would be one thing to do that at night, but totally different during the day.

I can do 73mph. I could easily have the speed set to 99mph (I think that's what they said), but I know I'd only use it for evil. For example, the only times I ever really want more speed is when I'm trying to bottleneck someone.

Probably, but we've also probably got different trucks. I've got a 2007 9400i with an ISX at 980,000 miles. I also pull a flatbed, and I put that throttle to the floor wherever I go.