Later model 3.8 swap in an "A" body

res1vw21 said

I am getting 20mpg hwy beating the crap out of a '71 318 with 110xxxmi, '71 2bbl cam, 302 heads, a holly 4150 650DP, edelbrock performer, hooker long tube headers, upgraded to stock orange box, OD4spd, 3.55SG 8 3/4, '71 swinger.

It's not the fastest car but it gets out of it's own way nicely.

I am going to do a mileage build 318 and shoot for 25-30mpg :)
Think higher comp + same small cam and spread bore carb; and 8 1/4 instead of 8 3/4.

Never been a big fan of the edelbrock carbs, lots swear by them I personally have never been able to get a proper tune, that being said I have one to try on my mileage build and I have a spread bore 650DP Holley to try too.

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According to Larry Shephard, the stock 360 cam was the best to use for fuel economy (252 dur and .410 lift I believe). throw in a set of Rhoades lifters and a good dual plane intake and 600 -625 cfm vacuum secondary carb. Keep in mind if you switch to 340/360 heads you will lower compression. I used a set of 10.5 compression forged pistons with 360 heads and got final compression of 9.2 in my 17.75 econo engine. also go for a low gear like 2.76. also try lock up trans/converter.

My dad had a 76 Charger back in the day and It got 20 MPG stock! I've never been able to break 20 MPG yet. If a stock 360 Charger can do it, it is possible. One of these days...

My setup was idling at 24" mercury with stock 318 heads, and would overheat over 80° outside. Problem was too much compression. Put on 360 heads and got 22.5" mercury at idle. Was able to use a 20.5 power valve and vacuum guage to keep power valve from opening whenever possible. Could still accelerate well at 12" mercury to stay "econo".

Just a few tips to help you.