I bought my truck used and it came with a cap (topper). A nice one too. However we found it restrictive for certain loads, anyone could look in and see what was in the bed, and it restricted visibility. We sold it on Crag's list and after a lot of research we ended up with an
Extang Solid Fold cover. The only other one that was on my short list was the
Bakflip. I decided I didn't like the Bakflip's panel against the back window nor the need for the rain gutter rails.
We use our bed like a car trunk most of the time. And I wanted relatively good security and the ability to remove the cover if I needed the whole bed for hauling gravel or dirt. I bought it last April. Aside from being a little heaver than I thought for installation and removal, it's been excellent. Maybe 10 minutes to install the first time, just to line it up and see how it works. Only 3 minutes to remove it, and 3 minutes to put it back on. Folding it back takes seconds. We remove it to carry our bikes in the bed and stick in the trailer, then put it back on when we arrive at the camp ground. Also it's very strong and you can stand on it, which actually is a handy thing from time to time.
Since I only drove it a short time with nothing on the bed, I can't comment on mileage.
Disclaimer, that's not our trailer in the first photo.