At the end of 1940, SR 166 was designated from Carrollton to the SR 92 intersection northwest of Fairburn (this intersection currently also carries SR 70 and SR 154, as well). By April, it was extended across the Chattahoochee River and paralleled SR 41 to the east as it entered Atlanta on Campbellton Road, and intersected US 29 at Dill Avenue, farther north than it does today. Prior to the next year, all of the Fulton County section of SR 166 was paved. By the end of 1946, SR 41 was redesignated as SR 154. SR 92 and SR 166 traveled concurrently (as they do today). SR 166 crossed the Chattahoochee River farther to the north than it used to. SR 154 and SR 166 traveled concurrently from just east of the river to about the current SR 6 intersection. By the beginning of 1948, the southern branch of SR 8 was redesignated as SR 8 Alternate. Also, SR 166 was paved from the northern SR 92 intersection to the Chattahoochee River.