The Collaborative International Dictionary of English v.0.48:

Overrate \O`ver*rate"\, v. t. [imp. & p. p. Overrated; p. pr.
   & vb. n. Overrating.]
   To rate or value too highly; to overestimate.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

overrating \overrating\ n.
   a calculation that results in an estimate that is too high.

   Syn: overestimate, overestimation, overreckoning.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

overrating
    n 1: a calculation that results in an estimate that is too high
         [syn: overestimate, overestimation, overrating,
         overreckoning]