Double post.

 

That's a magical truck. Don't ever trade or re-flash this one. It's one of the special ones.

 

That smaller part of the filter is a bypass section intended to allow airflow if the filter becomes heavily restricted. Personally, I have never been convinced that a reusable filter provides any real benefit and often provides opportunity to cause problems and most if not all of them have been proven to be less effective than the OEM/paper filters at their primary function: protecting your engine from dirt. Washable or performance filters? Waste of money. Unless your truck is modified and you race with it performance filters are useless on a stock diesel truck. There are a lot of people making a lot of money off of you people with claims and promises of improving this, preventing that or saving you money. All I will add is just be cautious. There are usually alternatives to the stock design - knowing what to change and why and what the intended or expected results are is crucial in making such a decision. In this case, I believe a filter change is not the answer.

The discussion of your Filter Minder gauge suggests that your filter is not restricted and does not need to be preplaced based on that. I doubt this has any affect on your current fuel efficiency concern as already suggested. If you have a verified significant drop in fuel economy I advise you to visit your favorite Ford dealer to have them look at the truck. Just be sure that you have not made any changes in how or where the truck is being operated that may be causing a drop in fuel economy.

 

Ah yes, I forgot to comment on your fuel economy loss OP. The DOE (Department of Energy) or some other government lab (like ORNL) actually did some tests to verify if a clogged air filter affected fuel economy. They used a Dodge Ram as their test bed for an HD pickup and ran it on a dyno to keep loads consistent. What they used to simulate a clogged filter was to stuff tons of rags into the intake to cause a significant blockage of air flow. What they found was that the system only reduced ultimate horsepower and kept injecting fuel at the rate the air flow could keep up with. This in turn meant that they had no loss in fuel economy. But they did lose ultimate power. They also found this to be the trend for a diesel BMW 3 series and gasoline powered cars as well. So a dirty air filter will no longer cause a loss in fuel economy with modern vehicles. Just a loss in power (depending on severity of clogging).

 

I don't know why this concept isn't an automatic conclusion? The ECM in virtually all new vehicles for the last decade is specifically metering fuel/air with great precision and obviously with less air available there would simply be less fuel injected in order to keep the engine running properly. So, yes, it would just have less power output.