Abstract: A harvester includes a separator configured to separate a cut crop into a billet material and extraneous plant matter, a billet loss sensor configured to generate a first signal indicative of the amount of billet material that is lost and unharvested at the separator; a trash sensor configured to generate a second signal indicative of the amount of the extraneous plant matter that is harvested from the separator, and a control unit configured to, based at least partly on the first signal and the second signal, determine a preferred ground speed of the harvester and/or a preferred separator speed of the harvester.