Leti-I'iga crossed for her first try in the opening minute and six more followed from the home side despite Saturday's rainy conditions in Whangarei, where the US will kick off their World Cup campaign against Italy on October 9.
Three victories in similar conditions over the last three weeks gave the world champion New Zealanders the title. Canada came second, overhauling a 10-0 deficit for a 22-10 victory over Australia in their third game.
The Black Ferns would have slipped outside the top two in the world rankings for the first time with a loss to the Americans but that never looked likely after scrumhalf Arihiana Marino-Tauhinu crossed for the second try after six minutes.
Marino-Tauhinu, playing her eighth Test, took Player of the Match honours for her performance, which included the neat inside pass that set Leti-I'iga on her way to her third try in the second half.
"I think the squad has gelled really well under our new coaching administration and you can see that out on the pitch, the girls really want to keep playing regardless of the conditions," said captain Ruahei Demant.
Former All Blacks assistant coach Wayne Smith was brought in as director of rugby after the toring Black Ferns were heavily defeated in four Tests against rising powers England and France late last year.
Smith will not have been pleased with the malfunctioning lineout on Saturday but knows he will be able call on some more experienced players, who were rested for series, when the Black Ferns go for a sixth World Cup title later this year.Â
