If you're going for fuel economy;
the principle players are;
cubic inches, rpm, AFR, cylinder pressure , vehicle weight, and tons of ignition advance.
If you are stuck with a certain pre-existing engine and body, that leaves just rpm, AFR, and timing.
Traditionally econo cars had 2.76s to set the hiway rpms at about 2200@zero-slip. Then the engine was tuned to exploit that. Even 8/1 LA318s can squeak out mid 20s mpg at that.
But along came the FMJ cars with 2.45s rears, and the rpm dropped ~11%. Then came 2.2 rears an another drop of ~10% So now we are down to 65=1800@ zero-slip. THat is a drop of 18%. It is generally accepted that your mpgs will go up ~1/2 of the % rpm decrease so .5 times 18% = 9% improvement in mpgs, that's a lot, and is your first go-to for fuel economy. I mean .9 times 20mpg suggests an increase of 1.8 mpgs.
But 65=1800 presents a problem for ignition timing. That engine is going to want a lot of timing advance to get peak cylinder pressure to occur at the right time. I'm gonna guess between 45 and 50 degrees, depending on how lean you can go. There is no easy way with just a distributor, to give a 318 that much advance, and still have the correct amount of power-timing the rest of the time. And so, you have to be realistic with your goals.
As to AFRs well that is easy; just lean it out until it starts running crappy and then back up a bit.
At 1800, you might run into another problem; can your engine actually make enough power to locomote your vehicle at the speed you want to travel. Sometimes, you just need a bigger engine, to make the required torque.
And that is usually a step backwards.
But one thing you can do that is killer good, is to increase the cylinder pressure, as high as your gas will support without detonation.
And this is why most modern cars have gone to aluminum heads, because you can crank the pressure so high. My 2.4liter Orlando for instance is pounding 220psi. While your typical 318 at 220 would self destruct with iron heads.
But you can easily get carried away using pressure to make economy. From 8/1 to 11/1 might get you 8% better fuel economy, say 1.6 mpgs on a 20 mpg combo. But if your engine self destructs with detonation, it will get zero mpgs. So there is a limit to how far you can go.
But what pressure does allow you to do, is to swap in a smaller engine!
But by far, the killer of fuel economy is mass, and the faster you go,aero.
The heavier the vehicle the more fuel it needs to maintain whatever speed. And the faster you push it, the more gas it it's gonna burn plowing thru the air.
So forget a slanty in a graintruck to do 75mph, lol.
You need a lightweight aerodynamic vehicle with a small,hi-efficency, low-rpm engine, a clutch, and to cruise at a very low rpm. Additionally it needs to be fuel injected and have a timing computer.
EGR you can leave in the garage.
But if you already have a certain package, the first go-to is always a tune-up, which includes a compression test and a valve adjustment if applicable.
If your pressure is down at 120 psi, with no obvious leakage, then there is not much you can do for economy, except to sloooooow down.
