Francis Grunow found this fun map and shared:

“That map is a Sanborn Fire Insurance Map from sometime between 1915-1920. It’s weird because the population it lists is for 1920, but it doesn’t seem to correspond with the Census figure from that year. Also, the boundary for the city it depicts doesn’t show annexations in 1916+. I just colorized it with GIMP. So it’s probably not a specific date, since Sanborn’s were additive and periodically updated”

The 1920 Census lists a population of 993,678 for the City of Detroit. Check annexations here.