Miles per gallon (MPG) Truth or myth?

[ktztrk]

I have heard that our trucks give low miles per gallon, even when stock and carb is tuned well.
I have a 1970 C10 350cid with turbo 350 transmission that burns the fuel very cleanly, has great pickup and gets only 13 miles per gallon.
The only problem I have is the kickdown does not engage. I don't normally need it or use it, but am working on getting it fixed.
Is the low gas mileage the truth or a myth?
P.S. I switched to a Holley 4360 Economaster carb from the stock Rochester
Quadrajet.
Any clarification greatly appreciated.