A chip should be able to net some significant MPG, I have watched the timing curve on my scanner on a lot of these rigs and base idle timing is frequently around 5* advance, WOT hardly sees over 25* and cruise conditions are lucky to get 30*. They seem to have killed the timing to get reduced NOX emissions. I have seen significant MPG increase from advancing the timing, you have to slot the crank sensor bolt holes and slide the sensor to the pass side of the block, you can get about 2* just doing that, you can get a couple more degrees by making the notch in the bell housing a little bigger in that direction. This will advance the timing through the entire curve. If you slide it enough to get 4* you may need some big washers under the sensors hold down bolts.

You can also use fuel injectors from a 4.6 or 5.4 ford, they atomize the fuel waaaaay better so the burn is faster and more complete. Just find a set of yellow injectors on an old ford or lincold in the JY with the same connectors, those ones work in the 92-99 magnum motors.

Another option is a small duration cam that builds cylinder pressure to speed up the burn.

Another problem is the air gets so hot from the intake belly pan that it retards the timing real bad as IAT goes up. Even with 40* ambiant air temps you will typically see 130-140* air flowing throught the runner. Try one of them lifter valley heat shields, an airgap intake or maybe a ceramic coating on the valley pan, anything to keep the IAT down. I tried a 2000 5.9 ram a couple years ago and in the winter with 10-20* air temp it was showing 120* after driving for a while then put on the complete K&N cold air intake and could squint real hard and see maybe 5* cooler intake temps.