How much power is lost through a 518

Somewhat of a topic-update;

I made a real attempt to get a good mileage number again recently.
I had to go on a 2hr one-way trip so decided to drive the same speed as I drove years back when I got my car and managed 17mpg with the low cr 318/904/7-1/4" in it.

Well, after driving like a senile old fart for some 4 hours at a max 60mph and become the most hated guy on the road, it turned out the car managed 16.5 mpg.
With some more air in the tires I'm sure the 17mpg could have been reached again.
But, during this trip the drivetrain now consisted of a 11:1cr 360/A518/8-3/4...