Last time I had a squeaking backing it up, it was the rear brake pad squealers rubbing on the rotor creating a sort of high pitched nails on a chalkboard kind of sound. It wasn't constant, just one 1/2 second squeak per rotation.
As for the fuel cleaner, I would't bother unless you are having fuel issues.
AS for fuel economy, run synthetic oil, keep up on your maintenence, run non-ethanol premium fuel and keep your foot out of it... ( that last one always seems to be the hardest....)
