Lance Armstrong -- How Much Did Sheryl Crow Know?

Lance Armstrong How Much Did Sheryl Crow Know?

Seems everybody in Lance Armstrong's world knew the truth about the cyclist -- that he was cheating his face off -- which begs the question ... how much did Sheryl Crow know?

Armstrong dated Crow from 2003 'til 2006 -- when they broke off their engagement. During the relationship, Armstrong rode in the Tour de France twice -- and won both times.

During Armstrong's interview with Oprah, he admitted to using every single PED under the sun and blood doping ... and acknowledged that everyone who came after him for cheating was right.

Which brings us to Sheryl ... who reportedly was interviewed by the feds in 2011 and allegedly provided info in the doping investigation. The Wall Street Journal reported ... two sources claimed she knew a lot about the doping situation. Crow has never spoken publicly about the matter.

We reached out to Crow's agent, who didn't have a comment ... and we tried to find a publicist for Sheryl, but were unsuccessful.

So, we gotta ask ...