The average rate the federal government actually pays on the debt it already owes, against nominal growth in the same fiscal year. Below growth, the debt ratio falls on its own.
Federal Reserve Board · OMB · CEA · BEA via FRED · net interest over the debt the year opened on, nominal GDP growth, and the ten-year yield, by fiscal year · change computed here
This page draws an interactive chart and needs JavaScript. The data is published by the US Treasury and the federal statistical agencies, most of it obtained through FRED.