In response to the question of the fuel mileage, I can give you some fairly average numbers from the two trucks I've had where I work now.
Our average haul is east of the MS, mainly in the north-east, carrying an average load of 40-45k lbs. All of our trucks has the Webasto heaters, so until it starts warming up, idling the trucks (lowering the mileage) isn't an issue.
When I started here, I was assigned a 2001 Volvo 660 with a Super 10. Over a period of 2 months my average mpg was 5.9.
I've been in my 2002 Volvo 780 with the 10 speed automatic (it's a former USXpress truck) for about 1.5 months now. My average mpg is 7.3. Also, I used to drive for USXpress, and those numbers are pretty much right on from when I drove for them.
So, figure an average week of 2500 miles (I honestly don't know my actual average, but 2500 is about average overall).
2500 miles @ 5.9mpg = 423.73 gallons
2500 miles @ 7.3mpg = 342.47 gallons
81.26 gallons less per week in the automatic
$203.15 less in fuel per week in the automatic (figured @ $2.50/gallon)
Pretty hefty savings...
As far as automatics being heavier than standards, it is. By about 30-40#. The only difference is that there's an additional computer, a servo plate with 2 electric servos that mount on the top of the tranny (replaces the gearshift plate in a standard) and an electric servo for operating the High-Low range in the tranny. Oh, and the control pad in the truck (R,N,D,L).
Before I received my current truck, the tranny had an oil pump go out, which sent metal through the gearbox, destroying it. The mechanic pulled out a newly rebuilt standard 10 speed transmission, put the parts I mentioned above on it, put it in the truck and it was done. He had no idea that they were IDENTICAL.
Myself, if I ever would buy a truck, it would be an automatic.