I usually get around 21 MPG (which includes a good bit of mountain driving) in my PIU with the 3.7l. Some folks' feet are heavier than others, and these Interceptors have very sensitive throttles, but substantially worse mileage without an explanation, such as being stuck in stop & go traffic for hours, would be something I'd investigate further, there may be something wrong like a failing sensor.
A few weeks ago, I picked-up an Ecoboost Interceptor Sedan that was hit from behind. It was to be an engine parts donor car for the Ecoboost PIU I'm repairing, but it drives straight, was cheap, and only needed a few things done, so I started driving it to keep miles off my other vehicles. I think it would probably average at least 26 MPG if it wasn't so fun to accelerate quickly. Folks have mentioned that you can have "Eco" or "boost" from an Ecoboost, but not both, and that seems accurate.