Some,
to a lot,
of fuel mileage gains
can be found in the cruise timing. Sometimes (depending on the cam) it is possible to run the engine at too low an rpm, and to not be able to give her the ignition timing she desperately wants.
For instance, if you gear your car for 65=2000, and the engine wants 52* of timing...... how are you gonna meet that need? With factory parts, it is nearly impossible. What if your cam is big enough to still be in reversion at 2000? What if, because of that reversion, your Effective Dynamic Compression ratio is just 4 or 5/1?
It's all in the combo, and especially in the ignition timing.
A manual trans, traditionally, has been cited to make about 10% better fuel economy, cruising at the same rpm. Part of that is in the TC slip; and that is why the factories have gone to loc-up convertors. And another part is inside the autos themselves. And a third part is in the transmission ratios.
Another major factor in fuel economy is your running cylinder pressure, which is governed by your Dcr (Dynamic Compression Ratio), the Ica (intake closing angle), and the throttle opening. This runs in conjunction with your carb, and carb size. If you gear your cruise rpm too low, and your throttle opening becomes too large, then she will get up on the mainjets. But now, the airflow thru the venturies will be quite low, because the power-requirement is so low. And it won't take much error in MJ selection to totally negate, or worse, all the work you put into the low-rpm running.
I used to run hi-compression 340s for years, beginning in 1970, in hi-school. 340s were never known for fuel economy except to say it was lousy compared to any other Mopar offering. And part of the problem, perhaps most of it, was the factory 268/276/114 cam.
That's part of the reason, as a streeter, I switched to 360s years ago. And the biggest reason I installed alloy heads on mine, was to get the super-high cylinder pressure..