Map of US highway 66 and family, and US 400

Map 1 (of 2) below shows the "earliest" US 66 and the routings of its original branch highways:

US 66 (historic) is represented by the red line from Chicago to Santa Monica CA. Its original route through central New Mexico is shown, as well as its original route between Springfield-Joplin MO and between Kingman-Topock AZ.

US 166 is the blue line from South Haven KS to near Joplin MO.

US 266 is the blue line from Warner to Henryetta OK. The red line extending west from there to Oklahoma City is the historic extent of US 266.

US 366 [I] (historic) is the red line from Amarillo to El Paso TX.

US 366 [II] (historic) is the red line from Albuquerque to Willard NM.

US 566 (historic) is the red line from Hondo to San Antonio NM.

US 666 (historic) is the red line from Cortez to Gallup NM.

There were numerous changes to US 66 and its branch highways over the years; compare the map above to Map 2 below, which shows "later" routings:

Map 2 (of 2) above shows:

US 66 (historic) is again represented by the red line from Chicago to Santa Monica CA. However, its later route through central New Mexico is shown, as well as its later route between Springfield-Joplin MO and Kingman-Topock AZ.

US 166 is the blue line from South Haven KS to near Joplin MO. The red line extending east from there to Springfield is the historic extent of US 166 (it was extended along former US 66 when that route was re-aligned along a more northerly corridor).

US 266 is the blue line from Warner to Henryetta OK. The red line extending west from there to Oklahoma City is the historic extent of US 266.

US 366 [I] (historic) is gone, having been swallowed up by extensions of US 60, US 70, and US 54.

US 366 [II] (historic) is the red line from Willard to Moriarty NM; by this time its former segment between Albuquerque and Moriarty was part of US 66.

US 466 (historic) is the red line from Morro Bay CA to Kingman AZ.

US 566 (historic) is gone, having become part of US 380.

US 666 (historic) is the red line from Monticello UT to Douglas AZ (its maximum northern and southern extents).

US 400 is also shown on this map (the green line from near Joplin MO to Granada CO). Because its number is a major violation of US route numbering conventions, I included it on this map to demonstrate how the highway would fit into the US 66 family, if it were to be assigned a number US x66. (This is just one possibility that makes more sense than 400; I've also included the route on the maps for US 50 and US 54).