overestimate

(noun) an appraisal that is too high. Synonyms: overestimation, overvaluation, overappraisal.

(noun) a calculation that results in an estimate that is too high. Synonyms: overestimation, overrating, overreckoning.

(verb) make too high an estimate of. Synonyms: overrate. Example: "He overestimated his own powers."

(verb) assign too high a value to. Synonyms: overvalue. Example: "You are overestimating the value of your old car."