Gas mileage....... it's all in the tune.
My 03 Dodge Ram used to have a standard 5.9 Magnum engine with factory tuned PCM. The gas mileage was awful.
I had a 408 built and installed a Vortech supercharger. Had Flyin' Ryan Performance create a tune and installed it into the PCM. I get better gas mileage with the bigger engine but also more horsepower and torque.
Now if I could keep my foot out of it then I would really know how much better gas mileage I'm getting.