Peyton Manning's single-season passing TD record is TOAST ... 'cause Patrick Mahomes is driving his Chiefs bus straight through it -- so says K.C. legend Tony Gonzalez.
Mahomes has lit up the league so far ... chucking 13 TDs in 3 games -- and Gonzalez tells TMZ Sports he thinks Pat will catch Manning's all-time mark of 55 in a single season.
"Right now, I'm looking at it, yeah," Tony tells us. "He's on his way!"
In fact ... the legendary tight end's been so impressed with Mahomes -- he's got him as MVP of the league for now .