What Ramman said^^^^. I'd start with unplugging the tuner and disconnecting the battery to reset the computer. See what happens and progress from there.

 

After a new O2 Sensor, Spark Plugs, and PCV valve, I reset the ECM (no tuner and unplugged over night). After extensive testing, it's mileage has bumped up to 11.6, which is an improvement. I was hoping for more. It never has the check engine light come on and there are no codes pending. When I drive, I never exceed 2000 RPM, perhaps that is too high and I need to slowdown even more? Do you guys have any other ideas?

 

What brand O2 sensor did you get?

I found that my truck actually got better gas mileage if I DIDN'T baby it...... I don't think it really made any difference at all. Just drive normally. The computer will learn your driving style. (which may be part of the problem.)

 

I listened to you and got a NTK. My Dad drive like a bat out of hell and it drops to about 9 or 10 when he does that. It could be our differing driving styles, so I will give it a shot. The thing sounds awesome anyway

 

Run some fuel injection cleaner for a couple tanks, maybe bump to premium gas for a tank or two as well. Maybe clean things out some.

 

Aha! So it isn't always 11.4?!?! If you're dad can make it have 9-10 then it's running normally. Some trucks can just get worse mileage than others. When was your last road trip ? The 5.9 gets one less mpg than the 5.2 (which could be attributed to the optional manual trans with 5.2) but I see 13 city and 15-17 hwy. You're not far off at 11.4. Go on a trip on hwy with cruise set at 65 and drive til the light comes on (should be about 300 miles) then tell us what you got. And if you go over 70, then don't whine about mpg anymore.

 

Last time it got 11.4 at 55 mph. That was a few weeks ago. It gets 11.4 going 70 and 80.