ZZman, just like your Grand Marquis, I always drive to save MPG with my 4.6 F150. 19 MPG is not uncommon.
However, I have a gooseneck ball in the bed, and when I am hauling lumber, equipment, or steel I make her work - but almost never at full throttle and really it never sees over 4500 RPM just about ever.
At the other end is my 4.6 DOHC Mustang Cobra, which sees redline regularly when I want to "play". It has 4.10 gears and sometimes I wish it had 4.56, LOL!
My 7.4/454 K3500 Chevy has gobs of torque. It gets absolutely horrible gas mileage. I only take it out when I need to pull a 40 foot gooseneck trailer.
Then there's my poor Chevy C50 truck with a low-compression 2bbl 350.
I think it weighs 12,000 lbs empty (all 6 tires/wheels must be over 1000 lbs just by themselves, LOL!) - and it accelerates just fine empty (2 speed rearend with pretty low gears) - but when its loaded I just leave my right foot flat on the floor 100% of the time unless going downhill!