The Collaborative International Dictionary of English v.0.48:

Overestimate \O`ver*es"ti*mate\, v. t.
   1. To estimate too highly.
      [1913 Webster]

   2. Hence: To overvalue.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Overestimate \O`ver*es"ti*mate\, n.
   An estimate that is too high; as, an overestimate of the
   vote.
   [1913 Webster]

WordNet (r) 3.0 (2006):

overestimate
    n 1: an appraisal that is too high [syn: overestimate,
         overestimation, overvaluation, overappraisal]
    2: a calculation that results in an estimate that is too high
       [syn: overestimate, overestimation, overrating,
       overreckoning]
    v 1: make too high an estimate of; "He overestimated his own
         powers" [syn: overestimate, overrate] [ant:
         underestimate, underrate]
    2: assign too high a value to; "You are overestimating the value
       of your old car" [syn: overvalue, overestimate] [ant:
       underestimate, undervalue]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "overestimate":
   aggrandize, amplify, ballyhoo, build up, burlesque, caricature,
   carry too far, draw the longbow, exaggerate, exaggeration,
   go to extremes, hyperbolize, idealize, idolize, lay it on, magnify,
   make much of, overappraisal, overappraise, overassess,
   overassessment, overcalculate, overcalculation, overcharge,
   overcommend, overcount, overdo, overdraw, overesteem,
   overestimation, overlaud, overmeasure, overpraise, overprize,
   overrate, overrating, overreach, overreact, overreact to,
   overreaction, overreckon, overreckoning, oversell, overspeak,
   overstate, overstatement, overstress, overvaluation, overvalue,
   pile it on, puff, stretch, stretch the truth, talk big,
   talk in superlatives, tout, travesty