i had an E58[cop] 360[stock with the exception of a 440 thermoquad], 727 with factory histall converter[2500], 8 3/4 [3.91] and L-78 x 15 rear tires in my 38. [pic in other post]
the car weighed in at 3485#. ran the 1/4 in 13.99 @97mph and got 18.5mpg on the interstate @ 70mph. not too bad for the early 80's. if a guy had a 518od., mpg would probably be in the 20,s especially if you used a lockup converter.