The official DaimlerChrysler press release said the 2004 5.7 Hemi was
“6% more fuel efficient than the Magnum 5.9V8”

They did not explain whether that was
6% more fuel efficient at peak torque (traditional SAE comparison)
or 6% more fuel efficient in the (somewhat crazy) EPA dyno fuel economy test.

That odd year I believe you could get either engine in a Ram, but auto trans differed.

With the roughly 55 year difference
I would guess 0.5% per year improvement
between a 440 V8 and a 7.3 V8
but the super strict NOx emissions the 7.3 has to meet is a “heavy burden”
and the ethanol in today’s gasoline has to be factored in.