In rough finished holes, they won't go to 1/32" increments. The self-tapper sizes the hole correctly for the thread, where your existing hole won't fit a 1/4. I'm operating under the assumption that these are threaded in sheet metal.
Judging from the one hole I drilled and tapped through the tailgate, there’s a welded nut on the plate through which the screw goes. Perhaps I can get a picture.