I use 5th all the time.  Only down to 4th or 3rd if I really wanna maintain 75-80mph up a decent hill, or pass someone quickly.  At 75mph I'm doing about 3k-ish RPMs.  Maybe a little higher, I can't remember.  But you know the 3rz loves to run.  Mine loves running through the revs...  And i get about 17mpg.  Whether I'm running 60 or 80.  I did manage to draft a semi a bit one night and got 21.5mpg.