Grateful Dead Reunion: Will John Mayer Be Playing With the Band?

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John Mayer performs at day one of Pinkpop Festival at Megaland on June 7, 2014 in Landgraaf, Netherlands. 

According to sources, a trek with the Grammy-winning singer and guitarist is being planned for the fall.

The Grateful Dead reunion shows, five in total scheduled for June 27 and 28 in Santa Clara, Calif. and July 4-6 in Chicago, look to have a potential haul of $50 million just in ticket sales, according to Billboard's estimates, and could have sold six million more seats, judging by demand. The staggering payday naturally leads to an even bigger question: will the "Core Four" surviving members of the Dead -- Bob Weir, 67; Phil Lesh, 75; Mickey Hart, 71; and Bill Kreutzmann, 68 -- take the show on the road, or will the Fare Thee Well concerts truly bid farewell?

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Speaking to multiple sources, Billboard has learned that a fall tour featuring John Mayer is in the works. According to insiders, the trek is due to kick off in October, with the Grammy-winning singer and guitarist having already begun to jam in a rehearsal-like setting with select members of the group, chief among them Weir. Observers say he's ecstatic about the prospect. A rep for the band tells Billboard that it's "premature" to suggest any such outing is being planned.

Certainly, the 37-year-old Mayer would have his work cut out for him. The Dead's arsenal contains hundreds of songs and many more free-form solos, led traditionally by guitarist Jerry Garcia, who died in 1995. It's also unclear how Deadheads, the most devoted of the band's fans, would react to the lineup. Sniffs one insider: "It sounds like Ratdog, the sequel," referring to Weir's longtime side-project.

For their part, the core four insist that Chicago, which marks the 20th anniversary of the band's last stand with Garcia and will feature guest turns by Phish guitarist Trey Anastasio, Jeff Chimenti (keyboards) and Bruce Hornsby (piano), will be their final bow... as the Grateful Dead.