This should join the ranks of the Unicorns. Already been beaten like the dead horse it is.
Bottom line, The Carburetor is only a small piece of a very large efficiency puzzle, you still have hugely inefficient engine, drive train, and aerodynamics to overcome. The engine in most cases burns 99% of the fuel, so there is very little to be gained by improving that part of the equation.
You don't think for an instant that if a car company could get the edge on its competitors by offering a 200 mpg car, that they wouldn't be all over it? It's delusional to think that the main car manufacturers get together in secret and say, "Let's just keep fuel efficiency down to 40mpg or less, we'll all make more money that way."
200 MPG Carburetor Talk