My 440ci Magnum in the 70 Challenger R/T is mild, at best, around 400hp (up around 25hp from stock). I get around 16-18mpg with driving normally. I mean overtaking (foot to the floor) and driving how I want, squealing the tires, launching hard, cruising,
real world driving. I feel 20 is achievable IF I wanted, but, I will never drive THAT slow or consertively lol.
Any car no matter who builds it won't get good mileage results when you go out for a proper blat/drive, ie: "enthusiastically driving" in which case I'd probably get, at a guess, 12-14mpg, tops. Getting good mileage can be done when it calls for. I may just set out to get the best mpg I can one day but it's too tempting having power on demand in a classic muscle car like this lol. I have 3.91s with GV OD which run like 3.05 and 28" OD rear tires.