Will this interfere with my tonneau cover or bed cover?

Test-fit with your specific tonneau before final installation. The spoiler's profile is relatively low, but tonneau geometry varies—verify clearance at the tailgate hinge area to avoid binding when opening or closing the cover.

Does this affect stability at highway speeds?

Yes, in a positive way. By reducing the pressure void that forms behind the truck bed during highway driving, the spoiler helps stabilize your truck and can improve fuel economy slightly by decreasing aerodynamic drag.

Can I install this with the tailgate attached?

You can, but removing the gate first makes the job easier and gives you better access to the mounting points. Either way, mounting takes basic hand tools and no drilling—just clips and fasteners at the factory attachment locations.