I have only lived 13 years in the USA, so possible that my experience is not a true barometer of the state of interest inF1.
However in the tri-state area I have met many many many Americans interested in Autoracing, they liked and loved NASCAR, CART, F1, Daytona 24 hours, spec series, non spec series, mom and pop racing, Dale Jr, Dale Sr, Rusty, Lauda, Schumacher, Villeneuve, Andretti, Gordon, Andretti, Andretti, Patrick, Rahal....... and on and on and on.
Point is that those in the US, who like racing likes all forms of racing, they will go see what is available close to where they live, some will make a trek once a season to a particular race, like I do with my brothers in Europe now every second year to see a F1 race.
They all follow F1, and would love for F1 to be in the US, when ever there is a race it have more spectators than most of the Tilke dromes around the world.
A fan is a fan is a fan regardless of country.