What did the 66 stand for in Phillips 66?

Phillips Petroleum Company | The Encyclopedia of Oklahoma History and Culture.

On November 19, 1927, it opened its first gasoline service station, in Wichita, Kansas. Because this station lay near the new U.S. Highway 66, the company used “66” in its name. Then, in 1930, Phillips began using the U.S. highway shield as its logo, making the brand easily recognizable across the nation.