well the old way was aiming for a highest vacuum reading at a desired rpm or speed.
highest vacuum means most efficiency. its the 55mph trick people used to do they would sacrifice a total tune in favor of a narrow RPM range to hypermile this was during the first opec embargo.
first you would want a Vacuum gauge on the intake manifold and see what the car is doing at the cruising rpm and speed you want. now you also figure the higher the rpm the more fuel you are using since the mechanical fuel pump is in line with engine rpms. an over drive in general will save mpg due to lower rpms.
so say with the overdrive your mph range is in the 2000s without it with the old 3 speed you might be 2500-3500 depending on rear end gear. so with the overdrive unless there is some drag from something you should be saving mpg
Now at steady cruise the only thing affecting fuel consumption is the Cruise Jets, so if you jet down 1-2 sizes and keep going as long as the engine does not surge then you lean out cruise and save mpg. your vacuum should go up also, because the engine is leaning out.
each time you touch the pedal the accelerator pump is going to shoot off, that needs to compensate for the overlap between the cruise circuit loosing vaccum and signal and the power valve kicking on, fast pedal movement is just going to turn the accelerator pump on and off. You can reduce shooter size up to lean fire miss and you have to come back up slightly. a secondary adjustment is the Cam that drives the accelerator pump there are different profiles to customize when fuel is delivered, again all depending on your engine, not what you want.
the power valve again can be swapped to come on later rather then early or staged to open at up to 3 different rates. stock the power valve is on or off usually at 6.5 HG vacuum.
The vaccum advance is dependant on engine load, so really you are talking about how fully in the mechanical advance is at a given RPM range. now Again... more timing means more power, if your seriously maximizing MPG then you want the mechanical to kick in as late as possible and pretty much never come fully on. Obviously the car will drive like a sluggish pig in that configuration.
so really you want a stock spring tension that Ford put on the car OEM because that gives the best of both worlds as originally configured. It also means you will want to be light on the throttle since that will burn up gas.
now this is theory because most cams are not going to tolerate this in the motor. your going to get carb backfire and a massive lean condition. and then you get into Fuel injection replacing the carb....
If you Truly want a 71-73 to get 35mpg, then you need to ditch the V8 for a inline 6.
also you need to ditch a secondary carb 4 barrel for a 2 barrel.
additionally a smaller carb will create a higher vacuum and reduced fuel consumption, again there is a lower limit to what you can tolerate. a 750DP is going to use gallons of fuel compared to a stock 550CFM carb.
Heads make a big diffference V4 verse V2 or Milled V2s where the ext and intake valves have been changed to the same size. 50/50 creates the highest mpg.
Headers are a bad thing, they lose heat from the combustion chamber and act like a timing advance of about 2-4 degrees.
another trick.. smaller exhaust higher back pressure will gain you mpg also, you also get a double dip from the carb fuel signal. so everyone puts a 2.25 exhaust for performance, go to a 2.125 or a 1 7/8s pipe and it will increase mpg at the cost of performance.
higher back pressure causes a bounce back wave in the engine. when the exhaust stroke vents it hits a restriction a backwards Air wave goes back into the engine up to the carb and Re-aerates fuel siting on the intake manifold floor that fell out of mix on the original fuel signal (vacuum) into the engine. that re-aerated fuel then goes back down into the combustion chamber for the power stroke, thus you magically got wasted fuel to be burned by the engine which means you can jet down, or reduce accelerator or power valve additional fuel because you already have more and thus boost MPG. this is called a Double dip,,, you double dipped wasted fuel and thus need less fuel from the carb at every rpm. This is what causes a MPG drop in a carburetor when you have an exhaust leak, suddenly the double dip effect stops and you get lean misfire so you compensate by jetting up or increasing pump shot.
When you have an exhaust leak in a modern Fuel injection the O2 sensor gets an incorrect reading and it sees a lean condition because air is getting into a sealed enviroment and thus adjusts more fuel in the injectors to compensate.
the engine is an air pump basically, you force more air in and out of it and it makes more power
you restrict the in and out and your air tank reserve(mpg) lasts longer.
you can't say at 2000rpms your timing must be 16 degrees, every engine is different and each day the environment changes, so you have to take the car out on the road get to a desired RPM and take a vacuum reading and a seat of your pants reading, if you have a oxygen sensor then you want the Air fuel information(you can retro fit a oxygen sensor computer on a carb car, i had one for a while on my 72), from there you say i want to save more fuel or make more power that that rpm, at cruise that is the main jets. if you want to go faster from a standing stop, then you need more fuel and more timing.
but the fun doesn't even stop with ignition changes and carb changes.
you still have final drive gear rations taller the ratio the lower the rpms at cruise the more MPG you save.
a 2.75:1 isn't going to do burn outs like a 3.25:1 but you will get 25mpg because of it.
transmission shift points and how fast a shift occurs effects mpg. you have heard of shift kits, well they trade comfort in gear changes for MPG, it has a performance aspect as well, faster shifts without lag increases mpg. also reduces clutch pack wear inside the trans.
remember the transmission vacuum modulator?
well that adjusts shift points based on engine load and it overrides the mechanical Hydraulic shift points, set it to maximize MPG so you get the engine to shift at low rpms through the gears. your acceleraion will suffer but your mpg will increase.
setting your kick down arm effects mpg how heavy will the pedal be for a shift down for passing.
so i mean you can go nuts
.
this is why you have a seat of your pants measurement, you can feel the changes you made and if the car feels really slow then you need to go back a little and trade off mpg for fun factor.