I've put 3300 miles on my Crestline since purchase.  I have tracked every fill-up and my overall average is 15.3 miles per gallon.  My odometer is GPS-verified to be accurate, so I believe this is real and honest fuel economy.  The past several tanks have all been almost exclusively highway miles (commuting to work around 65-70 MPH).  With overdrive, my tach is reading a little under 2500 rpm at this speed.  Economy on these tanks has been 15-16 MPG.  Worst tank I have ever recorded was 13.93, best 19.24 (once--thinking may not have filled the tank as full as usual).  

Does this sound about right?

1954 Crestline Victoria 312 4-bbl, 3-speed overdrive