318 4-barrel from the factory?

[Toolmanmike]

The objective is 20 to 25.

You might get 20 but 25 will take an overdrive transmission with a lock up converter if you even get there. That is really pushing the limits. Remember, the Dart Lite's

 

[Just Carbs]

In 1981 my healthy stock 1979 Chevrolet Camaro Berlinetta with 305 automatic (two barrel/four barrel?) repeatedly got 24 mpg at 65 mph. The car had 70k miles on it. Nice little car it was. A pretty bronze color.
I bet the aero dynamics were not near as bad as my 1975 brick Dart are. Carter two barrel.
The Dart has a fairly tall rear gear as it is, don't remember the number.
I want the 20 in town.

 

[RemCharger]

If your 318 engine had the California 4 bbl package, it was not lean burn. Lean burn was a joke and led to Chrysler going bankrupt in 1980. Driveability was crap and that "Lean Burn" mess was what made California recall the lean burn package.
The California 4 bbl package was a stoichiometric air fuel mixture that was just slightly rich - my group in California used the catalyst to clean up emissions while the slightly rich mixture provided good driveability, good fuel economy and performance. That box on the side of the air cleaner housing has nothing to do with air fuel mixture, it only controlled spark advance despite the label on it and the California package also provided a lot of spark advance for performance/fuel economy.

Lean burn was sold to the chief engineer because one ivory tower engineer who had more ego than brains said he could save Chrysler the cost of a catalyst by using lean burn to keep emissions low. Instead it led to Chrysler going bankrupt in 1980 and also cost Chrysler to have to add a catalyst anyway to fix the problem. I was there to witness it all going down.

The story I read from those times was the engineers wanted a fuel injector and the bean counters said " f*#k no! Those are a hundred bucks a piece!!!"