I believe I found the link that I was referring to here
View topic - Averaged 30 mpg! : PontiacBonnevilleClub.com the car that I was mentioning is owned by willwren.
The reason he mentioned two door is the GNs you mentioned to start the post is a two door.
The cars will handle much differently for city driving depending on the gearing and how it is driven. The mystery engine that you mention most likely was a manual trans with gearing optimised for low speed with one tall gear for the little bit of hw it gets.