Oklahoma City has no Beltway to speak of, but it does have a loop of sorts: the not-really-circular area enclosed within Interstates 40, 44 and 235. (I, as it happens, am out of the loop.)
There’s a sidebar to this Sunday Oklahoman story which defines the “inner city” as NW 63rd to SW 44th, Meridian to Martin Luther King/Eastern, a zone eight miles by seven with 160,000 of the city’s 541,000 people.
A term like “inner city,” of course, comes with all sorts of contemporary (or leftover-Sixties) connotations, not all of them necessarily pleasant. Still, this seems to be a reasonable approximation of what I’d consider the city core. I went back to the 1940 city limits, which are well within this zone: the northern boundary was around 36th, and the western edge of town was right around where I-44 runs today.
My preferred line of demarcation runs right along the original Grand Boulevard sort-of-circle, parts of which have been superseded by the present-day I-44. (The apparently-quiescent Criterion Group preservationists also used Grand as their boundary.) The disadvantage, of course, is that hardly anyone pays attention to Grand anymore; it’s just one more road that’s not on the grid.
That Oklahoman article itself, incidentally, deals with future development: at the present rate, the 600-square-mile expanse of the city will be pretty much filled up some time between 2050 and 2100. Population numbers are harder to quantify, but I think it’s unlikely we’ll end up with numbers like present-day Houston, slightly smaller at 580 square miles but already boasting two million residents.