sorry, i'll correct, thxOJ!

 

1. Oil changes are all done at the dealership. I use synthetic oil. I am assuming theyt use the right one.
2. I live up in Canada 22 degrees farhenheit maybe right now

 

I think these trucks might have a fuel saver on top of MDS? In my 2012 the "ECO ON" light will come on once i hit a cruising speed but still be on all 8 cyls.... then maybe 1 minute or so later i will feel it go into 4 cyl. sometimes it's still in 4 when the ECO light isn't even on, so idk if it cuts fuel flow to all 8 for econ then goes into MDS or maybe the ECO light just doesn't know what the hell is going on.

 

Ya i think the eco light (my truck says Fuel Saver) goes on just to entice people to save fuel, it's just there to look at when your easy on the throttle and probably help ease on the gas when driving around, kind of a mental thing because like you said it's sometimes still in 8 cyl when the light is on. I can tell a bit by how it sounds but there's no actual light or warning that says it's on, kind of stupid though. I know some GM's tell you when it's in 4cyl which is pretty nice.

As for the OP your truck is a 2009 and you said you don't have the fuel saver light? My truck is an 09 and i never used to have it when i first got it but they flashed my truck at some point and now i have it, maybe you just need to ask your dealer if you can get it.

 

First of all I would figure it out by hand. Second of all how long are you letting it "warm up" before you drive it? I reset mine before I make a 60 mile round trip to town n back 60mph max and by the time I make it home it says 22.5 - 23mpg on the gauge. I'll let it warm up for 5 min the next morning and the gauge says 15.5 - 16. Im keeping track of this tank vs. miles to see whats really going on.