Synonyms for Overestimate (same or very similar meaning)
WordNet sense 1 (an appraisal that is too high):
overestimation, overappraisal, overvaluationWordNet sense 2 (a calculation that results in an estimate that is too high):
overrating, overreckoningWordNet sense 3 (assign too high a value to):
overvalueWordNet sense 4 (make too high an estimate of):
overrateFrom the ODE community, based on WordNetadd/edit