[M_a_t_t]

I would probably just start with adding a panel (coroplast maybe, doesn't really matter) across the top of the arch as low as you can without rubbing over bumps.

Your wheels stick out from the bed right? A little hard to tell how far in the pictures you posted. You could then try and make deflectors to guide the air to the face of the tire instead of the tread.

A tuft test would be a good way to check for improvements. When I tufted the my car the front didn't have much turbulence behind the front wheel, but it doesn't stick out like yours.