If you are talking about t he chips you put in the fuel tank,I've got some beach front property in Az I'd like to sell you.If your talking of putting a different chip in the computer,thats a little different,but what that will do,is change the shift points of an automatic tranny,and change the fuel air ratio,this can raise hp,but will take away fuel mileage.
You want to save fuel,just drive like you have an egg under your gas pedal.
In a class 8 truck,if the ecm is changed so you have a HP increase,you can save fuel,as long as you don't use the extra HP,but as soon as you hit that big hill,and want to show off your power,look for a serious fuel mileage drop.
The crazy things that are sold at truck stops,just show,that a truck driver will buy anything,from deer whistles to pills for the fuel tank to get better mileage and HP