318 Magnum 70 Dart Gas mileage

Put a vacuum gauge on the intake after the engine is warmed up.In neutral, rev it up slowly and steadily, and watch the gauge. When the vacuum stops rising, record the rpm.Slow it down until the gauge has lost 1 inch vacuum.Record the Rpm. Drop the Rs back to idle.
The rpm of highest vacuum is the preferred minimum hiway rpm.It will get you the best MPGs
The second number should be considered the absolute lowest rpm for efficient running.
The numbers will change slightly with mechanical timing, but the actual rpm will only vary a bit.
It is very important to have a well worked out timing curve and a fully functioning vacuum advance system.
Typically, the vacuum will peak just off idle for a stock teener, to 2400 with a 360 and a big a street cam. 2000 to 2200 is a good number, but a cam this big is gonna suck gas around town, depending on your driving style.

Around town;
the 4.10s will not affect your city mileage much.Much more will your driving style than anything. 2200rpm will get you about 40/45 mph in top gear.Early upshifting with a big cam is not very effective as the engine is forced to operate in a very low efficiency zone. The headers can really upset things.
Of course the thing that will really burn up the gas is a poorly tuned low-speed circuit in the carb, and an over-compensating accelerator pump.

On the hiway, keeping the revs down is priority one. But There is very little to be gained by reducing rpms below the rpm of maximum vacuum.
Example; if your engine vacuum peaks at 2200rpm, then she will want to cruise not much less than that.With 26"tires,2200@ 65 takes 2.63s. 2.76s are the nearest, and they will cruise at 2200=62mph.
Of course 2.76s will be really soggy off-the-line.
Unless you have a near stock cam.

So the first thing I would do is a compression test, to see what we have to work with. With small number gears, cylinder pressure is a really big deal.
After that is to get the tune right.
After that is to change your driving style.
And finally after that is to swap out the rear gears IF hiway cruising concerns you.

Around town with a lot of stops and starts,those 2.76s may possibly actually cost you mileage, and the reason is, that every time you take off, you will be into the powervalves. And when it shifts into second and you want to speed up, you will be deep into the mains.
here is something to try.
With 4.10s and second gear, the road ratio is 5.945.
With 2.76s and first gear, the road ratio is 6.762
The difference is 13.7%
So go start off in second gear and feel the laziness, keeping in mind that the 2.76s will be just 13.7% less lazy,lol. The2.76s are not for everyone.
Here's another;
take your car up to 32mph,in second gear. This should be about 2450rpm.
With 2.76s and first gear, 2450 should be about 28mph. So once up to speed, there is very little difference around town.You are just using a different gear.
Of course there are several other compromise gears to choose from, as earlier mentioned.
When it comes to gears, you are always trading take-off-power, for hiway cruising. If you don't spend any time on the hiway, I see very little benefit to ditch the 4.10s.
If you do, or want to,spend time on the hiway, then the go-to gear is 3.55s for a 318. Perhaps the Magnum can suffer 3.23s. The compression test will help determine that.