Motivation or realism?
Peyton Manning's dad -- legendary QB Archie Manning -- says Las Vegas is DEAD ON with its Super Bowl line ... agreeing that the Denver Broncos are DEFINITELY underdogs on Sunday.
We got Archie arriving into San Francisco to cheer his son on ... and asked his thoughts on Peyton's crew being 5 point dogs going into the big game.
If you're expecting some fatherly sugarcoating ... you ain't gonna get it here. Archie says that line "sounds about right" but reminds everyone it'll be 0-0 when the ball is kicked off.
Tough love ... it works even if your son is damn near 40.