There's a way to fill with faces only the area between the larger circle and the the smaller one? I mean.. It's a wheel and I'm trying to fill the rubber part of it.
Using the fill function F it fills everything inside the bigger circle.
If both circles have the same number of vertices you can use the Bridge Edge Loops (from the menu Mesh > Edge > Bridge Edge Loops, or press F3 to bring up the search menu, and start typing bridge edge loops)
It will not fill as an NGon, that is generally not desirable anyway, instead creating an even quad-based topology.