318 MAX fuel economy builds?

So I'm looking into buying a long-distance capable daily driver. I have my eyes on a Dodge Diplomat and Chrysler 5th Avenue. Both have 318s and I thought it may be of my own interest to buy one as a daily as they are things I can work on instead of things I would need to take to a shop. In the case that I buy one or the other I would like to build the 318 for high fuel economy, I'm talking like 20+ mpg if possible. Anyone done this? Any input? Etc. Let's coffee-table talk daily stuff instead of high horsepower for once.