Gas mileage 225 Slant 6

If I've noticed one thing in all my years of driving, owning cars and trucks, and working on them for a living/
Most vehicles I have owned I have had "2 of" or more.
When I had my 1st XJ Cherokee, I had very little trouble with, very consistent and predictable 16/20 mpg, just a great vehicle.
After my son spun it 180* on a slick road and rolled it into a ditch during his 1st winter as a driver we replaced it with another, seemingly identical except for the color.
This 2nd one gave me electrical fits that the 1st one never did, lots of issues the 1st one never gave
And not as good on gas either.
Same story on the 2, ex police diplomats, the 85 was great, so much so we then has an 89, which was a pile of junk. And lather rinse repeat with the several other vehicles I've had over the years.

I see a wide variety of results for the same questions on different /6 vehicles here.... hardly 2 of the same stories among them... Could be "built on Monday" (or not), advance in distributor not working the same between them, different gearing, different driver routes, different clearances within the engine, dragging brakes on some and not on others, different gas quality, who knows?