74 Dart. Headers vs manifold.

Headers aren't going to improve your gas mileage. They probably won't hurt it either, but if it's an otherwise stock 318 they're not going to change a lot for you. Maybe a small increase in mid-range and up horsepower, but probably not something you'd notice.

And unless you're going to buy a set of TTi's or Doug's D453's, headers are probably going to be a much bigger headache than any benefit you'll see.

What's "****" mileage? With a relatively stock 318 you should be able to get 15-18 mpg depending on your gearing. If it's any less than say 12-13 mpg you probably have a tuning issue, unless it's got a big cam in it.

We'd need to know more about your engine and gearing set up to help much more than that.