State Route 166E was a renumbering of State Route 202, which was first designated in 1925. When US 202 was designated in Maine in 1934, it became a duplicate numbering but was not renumbered until 1939. SR 166E was the longer of the two split routes, serving Castine to the south and connecting with SR 175 to the north. This designation only lasted until 1941, when SR 166E, SR 175 and SR 199 underwent routing changes. The remaining length of SR 166E was renumbered as SR 166.Floodgap Roadgap's RoadsAroundME: Maine State Route 166E
