Is there a scientific chart showing how much fuel you save by going slower?

No because each truck is different, it isn't like a car where you have the same exact drive train and the same exact engine in each one.

The closest is an estimation from an old formula.

 

It's really a matter how much lead you have in your foot

 

You could make such a chart for a particular truck, but not for all trucks as all of the specification variations can make a significant difference.
The Europeans ran on a test track and found that the absolute best fuel economy was obtained at 70 kph (45 mph), somewhere around 13 mpg. And it's all downhill from there. It's mostly about compromise, you can gear a truck to get the best fuel economy it can at 70 mph. But a truck geared to get it's best fuel economy at 55 mph, will get better fuel economy at that speed than the one geared faster and going faster because of aerodynamic physics.
Also you pulling over, stopping and passing that slower truck four times a day means you just wasted a bunch of fuel getting back up to speed four times. In a short haul operation going faster may actually work out better to get an extra load as you suggest. But in long haul the math doesn't work out in the long run.
For your reading pleasure: http://www.theicct.org/sites/default/files/publications/HDVemissions_oct09.pdf
"The overall fuel savings caused by reducing maximum route speed from 70 MPH to 55 MPH is 7.3 to 8.1 percent, depending on whether or not the final drive ratio is adjusted"

 

I think they say .1mpg for every mph after 60 is was Cummins says j believe . I drive slow most of times but it a few reasons. First you don't have to worry about getting blocked in behind the slow person since your the one that's does 65 in the 70mpg zone. If I have a I have 2 days to deliever the load why get there the first day ? Also I have smaller fuel tanks so if I go fast and get 5mpg vs 6 mpg I'm going to lose what ever time I saved by going fast burning a hour at truck stop waiting in line to get fuel plus the added expense . It's a big picture do what works for you. Should mention I run a unrestricted truck I think wide open it be close to 90-100 mph truck when it hits the rpm redline . Also you saving on maintain economic cost all the way around it a lot less work for the motor to push 80,000 at 65 vs 75mph . Tire wear as well.

 

I ran my truck at different speeds to figure this out.
You will pass me a few times a day, I run 62 mph. That gets me into the 8's.
I run this slow for a few reasons, fuel, and just the wear and tear on the trucks.
Most of the roads beat the hell out of the equipment. Tires, suspension, etc.
I log everything at 66 mph and over 300 miles I will lose around 30 minutes, but I'm not as tired or stressed. Just keep the doors shut.
Also, my truck doesn't have a speed restriction on it besides my foot.

 

Ya but how are you going to run 62 and log 66 when Elogs become mandatory? Basically you cheat on your logs so you can save a penny and hold us all up on 2 lane roads and in heavy 4 lane traffic

 

so you drive as fast as you can and you pass the same truck 3-4 times a day.... so how are you making more miles...?

the BASIC rule is for every mile per hour you slow down you save .1 mpg... slowing from 68 to 63 (thats what i run) saves about .5 mpg...

 

I was thinking the same thing.... )

 

Relying on your instant dash guage for analytical research won't help you. A scan guage with trip monitors, and tank to tank filling calculations, over a 6-12 month period will yield better answers.

A direct answer to your question - YES - all major engine manufacturers have conducted intensive studies as to fuel consumption. I'm just not prepared to find them all for you.

As to your second paragraph - YES, I would agree - BUT, only if your freight is paying enough to justify the lost revenue in fuel to achieve a time gain in loading/unloading. Example - if you can unload or drop and hook when you get to the reciever, and reload - before your HOS or customer hours restricts you - THEN - it would make sense to haul donkeys to get there.

There's far too many variables. I should have also asked if you are looking at this from the perspective of a company driver who gets fuel savings incentives, or an owner operator.

FINALLY - There's a guy I've run with this year with a large car that goes everywhere at 72+ mph. I'm the one lagging behind at 63-66 most times. He was on a dedicated, but I would often fill in on the same route with him if freight dictated.
He get's 5.5MPG. He knows he could get 6.5-7, but just hates driving slow. I get 7.1 summer, 6.9 winter. I did the math and showed him the calculator at dinner one night, and said to him . . .

"You know how much more chrome you could get for that thing with $12,500 ?"