Fuel Injection - explain this to me like I am 6 years old

Well that makes it official, your tune is just out to lunch.
And if it's a fat carb, she probably ain't making any power either.
Has she still got the factory single exhaust?
What carb and what intake,and what type of air-cleaner housing are you using?, or what did it come off?

That mpg number you quoted; was that steady-state hiway? and; With what rear gears,and What roadspeed or rpm?
With 2.7Xs say,and a 904, and 65=2285rpm (26s), and a 1971 spec engine with minimal leakdown, your mpgs should be at least 50% better.
To get hi-number mpgs with a carb is as easy as leaning it out until it misfires or surges; Then figuring out why it's doing that, correct it, and continue until it melts or breaks something. This costs you nothing but time and dirty fingers, and maybe a few low-dollar parts.
Geared for 65=2285, I'm thinking 20 is easily doable.And she'll have still more left in her.But you'll have to feed her fresh cold air; and then there'll still be more left in her.