Hemi 60s Charger, 425hp gross, probably rated very low for insurance purposes, but let's say 425 net hp, and it probably got about 10 city, 13 highway
Hemi modern Charger, 425hp net, probably pretty accurate, EPA rating of a 2010 6.1L model says 13 city, 19 highway
Hemi modern Charger, 340hp net, this is the 5.7L model, EPA rating is 16 city 25 highway.
So if you want to match this pony for pony, The 6.1L engine if used with all the computer and wiring and sensors and matched up with the same transmission, tire diameter, rear axle ratio, and everything, should get the same or slightly BETTER mileage. After all, the '68 Charger weighs LESS than the '10 Charger, by 200lbs. This may be offset by aerodynamics?
But wait, there's more.
The new 6.1L Hemi weighs about 240lbs less than the original 426 Hemi. So now we're talking a 440lb weight reduction. Some of this may be offset by using the modern overdrive transmission and adding the computer and wiring. I wish I had specs for the 5-speed automatic in the 2010 Charger R/T but I don't. The old 727 Torqueflite was about 160lbs with fluid and converter, so it's pretty light and the WA580 is supposed to be light as well so let's say they're the same.
I say use the 5.7L version. Should be able to match mpg for mpg with the modern one if not slightly better due to reduced weight. Actually, I would be plenty happy in a 3895lb car, oops sorry, I mean a 3455lb car, to have all of 340hp on tap. The 230hp EFI 318 in my wife's Durango is pulling around 4100lbs of truck and it's PLENTY quick with a 3.55 gear. With 340hp and less weight, you could go so deep in gears, maybe a 2.94 or a 2.76, and still have all the power you need to maintain highway speeds.
My goal is to take that EFI 318 and 4-speed auto from my wife's Durango when she FINALLY gives it up, and drop it in a nice 3-seat Coronet or Satellite wagon, probably 68-69 for the Coronet, 68-70 for the Satellite. A 1969 Coronet wagon with a 318 and automatic was a 3688lb car and it's lower to the ground than a Durango. The 19mpg that engine gets in the Durango with 3.55 gears would probably be more like 24-25mpg in the Coronet considering it usually came with a 2.76 axle standard. A vacuum gauge, judicious use of brakes, planning WAY ahead, LRR tires, smoothing the underbody, smooth hubcaps, wheel skirts out back, synthetic oils and greases, and making sure it's fully tuned up could see 30mpg in theory. I had a tank once in the Durango pull 21mpg in careful driving with the AC off all the time and the windows up and never over 60mph. It CAN BE DONE.
can be done yea. BUT i feel your killing the way the charger was. I dunno i just have a soft spot for big iron v8's back then. so freakin simple and sound the best.
but yea aero wise the old charger body as a whole doesnt look that bad it just the flat front end(grille area) and the high ground clearance it has is what kills thier MPG i think. but they look oh so good!!!