Do these flares add tire clearance, or are they mostly for looks?

These are styled flares designed to follow factory body lines—they're not engineered for additional tire clearance. Use them for the Tidal look, not for fitting wider rubber.

Will I need to drill or modify my truck to mount these?

No drilling required. They mount at factory rocker and fender points using the pocket rivet system, so your truck stays unmodified.

What size tires will these work with?

These flares are designed for factory tire and wheel sizes on your Sierra—they won't accommodate oversized tires. Stick with OEM or comparable sizing.