Since beside legal issue, we discussed truck protection, I made couple of pictures.
Here is my RWD dually with camper on it. Camper is lightly loaded on the picture and rear fender ends about wheel centerline.

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Here is bottom of my fender. Fender flare is plastic and 2-tone has brown color that you see on the bottom.
This is the part you never see unless you stick your head under.
Regardless my share of driving on gravel, when I nicked my cargo trailer pretty bad, I don't see any rock chips here.
So I really don't see any need for flaps, unless for trailer protection.

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