Just a thought, check your MAF sensor. I have never seen the motor of an 07 and dont know what the throttle looks like. But I know when these sensors get dirty, it will make the transmission shift funny. When I descreened mine when i got it, 4 days later it burned up the transmission. When I would come to a stop, I could feel it bang a little like I had a motor mount loose or something. Then it did it when I took off. Very annoying. That sensor is very delicate. The cleaner it stays, the better ur truck runs.

 

I bet the 2007 4.3 is the same as the 4.3 that I had in my 96 blazer. I don't know why they didn't come out with a better v6 since they replaced the 350, which is the best engine in the world IMO, and the 305. Idk, every car company does a weird thing every now and then.

 

I have the same truck- 2007 Classic, 4x4, same engine and it's bone stock, even the tires (didn't want that engine but the truck was exactly what I wanted otherwise). I have 52K miles on it and have consistently gotten about 17 mpg no matter if I do around town or highway driving. I drive pretty conservatively so that may be why. The V6 is overworked in that truck, so any gas savings from a smaller engine are lost to the engine working harder. I figure if I sit all day idling in traffic there might be a small mpg advantage.