How to keep inter-regional gaps in higher-education development within a reasonable range is a shared global challenge, yet much of the literature still treats zero gap as the benchmark. Grounded in the education-led development perspective, we posit that a gap is “reasonable” when education disparities are smaller than contemporaneous economic disparities. Based on this concept, this study develops a Development Equilibrium Index (DEI) framework. Using panel data for 31 Chinese provinces from 2003 to 2020, we find the following: (i) N-DEI is positive in most years, indicating that, overall, education gaps are smaller than economic gaps—consistent with higher education’s leading role in compressing economic gaps and supporting sustainability; (ii) At the provincial level, only 4 of 31 provinces (12.9%) show negative P-DEI, while the vast majority are positive, suggesting more supportive conditions to sustainable development. Furthermore, a median-based quadrant analysis for 2020 groups the 31 provinces into seven descriptive types, helping to interpret the variations in P-DEI signs and magnitudes and to inform targeted policy recommendations. The DEI thus reframes assessment from “narrowing gaps per se” to the goal of keeping education gaps below economic ones, providing a concise diagnostic tool for planning cohesive, resilient higher-education systems.