Best possible mileage?

AJ/FormS said

The one single fuel waster is not having peak cylinder pressure occur in the window of 25 to 28 degrees AFTER TDC. Put yours there and see what happens.
I would like to tell you that I once did, and gearing my reasonably shaped 68 Barracuda to 85=2100 rpm with a manual trans, she returned to me 32 mpg. But nobody believes that so it must be impossible. And this with a 360 and a 223/230/110 cam yet....... so you didn't hear that from me.
My cruise timing? set to 2100 =45* in the distributor plus up to 15 more on the dash-mounted dial-back, spark-delay box, so 54* on this trip. That's what it took to put the peak pressure inside the window.

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I don't get 85=2100 rpm... what do you mean?
and I put a 252 cam (stock 360-2bbl) into an 83 D250's 318 and its difference was huge in power and mileage improved too. Never touched distributor advance curve (juust advanced it til it pinged and then backed it up 2*, stock original 83 distributor) but along with that cam, I
didnt degree it, just put it dot to dot with new Edelbrock True Roller chain
swapped from 2 bbl Holley 2280 and stock cast iron intake to Performer (not RPM) and 600 CFM CarterBrock carb
put on a set of EQ Magnum heads- bought new as bare castings and cherry picked best hardware from 3 sets of stock cracked Magnum heads, no porting done besides how they came out of the box
and a set of Hooker Super Comp headers, 2-1/2" true duals all teh way to the bumper from the collectors.....

How would I go about "putting peak pressure at 25-28*ATDC"? and is this a universal figure?