better MPG or trashed tranny?

[VICSVX]

so i hav been readin that are cars get the best MPG at 80MPH...so when i was drivin down I-10, i was doin 80MPH and looked at my tac, it was at 2700 ish....

so my question is since i do about 60 on Houghton (home, wrk, and skoo), would i get better gas milage poppin the tranny into 3rd....the tac is about 2700.

or would i get better milage but a trashed tranny?