The Minority Report star ditched his diet, grabbed a few pals and mapped out their trek using U.S. celebrity chef Guy Fieri's TV show Diners, Drive-ins and Dives as a guide.
He tells American TV host Jay Leno, "I rented a car in Georgia and we drove across the States, basically looking up on the iPhone... where the next site we were gonna hit was... (using) Diners, Drive-ins and Dives."
However, Farrell reveals there was one restaurant that stood out above the rest: "There was a placed called Nic's in Oklahoma that did a mean cheeseburger."
And the toned actor admits the trip wreaked havoc on his waistline: "I gained a s**tload of weight. I gained about 15 pounds in 11 days... By the end of it, I had a belly button like a compact disc... It was unbelievable."