How much power is lost through a 518

BigBlockMopar said
My Dart went from a 904 to a 518 (LU+OD) with no benefit in mileage. MPG even gots worse for a bit.
This was with a low power 8.6:1cr 318ci and 3.23 gears in the lightweight 7-1/4" axle.
First 518 went bad, changed for A518 w/OD only.
Changed rear axle to 8-3/4" w/3.23 gears, no change.
Changed to 3.55 gears, no change.
Changed engine to 11.3:1cr 360ci, no change.

I did notice a difference in revving the engine with the smaller lockup stall convertor. The other 518 got a large late '60s stall convertor.

What DID change was the car got way more fun to drive. The added 4th, OD gear really brings an old car into the present day, by being able to drive along modernday traffic with turning a silly high rpm.
I'm currently preparing an A500 to replace the A518.

If you think about that, it sorta makes perfect sense. With 3.23x .69=2.29; you're asking a smogger-teen to pull 65 at about 1840 rpm. That would require a completely different carb and timing tune, over the 904... which you don't mention.
As to the 11.3/ 360, well that takes a complete different tune as well.
With 3.55x.71x.78= 1.966 final drive (A833od/GVod), I got my 10.95 Scr 367 cuber up to 32mpgs, without hardly trying.
Now, I know you are a smart man, but I think you left out a few details,lol.
I was running a [email protected] cam and nearly 190psi cylinder pressure. The carb was an ancient Holley1850 set up strictly for this run. The gearing gave me 1850 @~75mph,right on the money. I put a lotta timing into her, a lot.