My 426 isn't anything special. Stock bore and stroke with 11.5-1 compression and stock iron heads with an M1 intake and a 750 CFM Holley along with a .525/292 cam. The car is an automatic with a stock 1800 stall convertor along with 3.31 gears.
From what I have driven the 1970 Challenger my milage is running about 10 MPG with mixed town/highway driving coarse there was a in there.