Re-reading this thread, there is an interesting point - Security or MPG.
So, given an open pickup bed provides no security for items in the bed, which is better, hard cover, tri-fold or soft cover?
Out of sight is out of mind. And cover begins to address the security aspect. At question is how much effort (and expense) to you want to encounter.
Personally, hard-cover and tri-fold are about the same for "security". A soft-cover is a step below. All have a "lock" but, what is your security comfort level?
If you favor a hard cover, plastic, fiberglass or aluminum are your choices as they are knife resistant.
At this point, my preference is an aluminum hard cover. I have had all three on various pickups over the years and without a doubt, I prefer an aluminum cover for all-around functionality.
It provides for security and it also provides for extra support for those times when you really need to tie a load to the top of the cover.
Fiberglass, plastic and other soft-top options do not have a load capacity that can match an aluminum cover.
Now, to add fuel to the discussion, all covers for pickup beds do add limitations for use of the bed for odd shaped cargo.
While mpg and security are important points, load carrying capacity is also an important consideration.
As noted, I have had a variety of covers on pickup beds. For MPG and security, all offer advantages.
For overall functionality, put a cover on your pickup bed. But, consider your various uses of a pickup bed before your final decision.
As I have had a variety of covers over the years, my current pickup (2019 Dodge 2500) has no cover over the bed. I have opted for an "in-bed" tool box that provides for security of items and still retains the functionality of an open bed for odd shaped loads. Yes, with the lockable tool box, I do lose some cargo capacity.
Is there an MPG improvement? That is open to question. As I have the diesel, I am averaging just over 20 mpg which is better than any other pickup I have owned.