From what I hear the chips don't work they simple mask sensors and can actually cause harm in the long run. What you need is a tuner which tunes the engine control computer. The stock program is quite conservative, the programmers allow you to optimize pretty much everything. I'm running the SC Cortex and noticed improved performance and MPG using the 91 tune.
I have yet to hear a confirmed report from someone using one of these chips and actually having an increase in fuel mileage. If you get one, maybe you'll be the first.
In the case of the tuners, I have read a large number of reports indicating the benefits are there.....HOWEVER.....
In my case (and I'm not alone here), I installed the tuner and have enjoyed the performance benefits SO MUCH, that I have experienced a rather substantial drop in fuel mileage. This is not really the fault of the tuner, but I will be the first to blame it for it's part in making my driving experience so enjoyable that after 1 1/2 years of owning the truck, I look forward every day to going for a drive.
Depending on the circumstances, a tuner can help you with mileage. Most of the time it WILL increase your smiles per miles index so much, that you will no longer care about the miles per gallon index. :smileup: