This website track each colleges' self-reported list of its peers.
Tufts lists the following as its peers. Most are clearly aspirational (i.e."These are schools we wish we were peers of'.").
Johns Hopkins U
U of Pennsylvania
Washington U in St. Louis
Of those above, interestingly only BC lists Tufts as a peer.
Below are those schools that list Tufts as a peer:
C of New Jersey
C of William and Mary
George Washington U
New York U
Southern Methodist U
U of Phoenix, Jersey City
U of St. Thomas
Wake Forest U
Worcester Polytechnic Institute
So who are Tufts peers? My suggestions are:
University of Rochester ++
++ most alike in terms of composition and finances
Interestingly Williams College lists these schools as its peers: Amherst, Bowdoin, Carleton, Dartmouth, Grinnell, Haverford, Middlebury, Pomona, Swarthmore, and Wesleyan. These make sense to me, While some are clearly not as highly ranked as Williams, they are all academically top notch and most are highly endowed.
More explanation here: http://chronicle.com/article/in-selecting-peers/134228/