Mpg 318

Something is not right , I get over 20 with a 340 n, you should be getting at a MINIMUM 15 mpg , my old 1970 440 New Yorker with a 440 got 12 MPG on the highway , the 75 440 New Yorker got 16 mpg highway .
It's not the crap gas of today , what gears you running ? What shape the transmission in ?