Well I have to add my 2 cents.
I average 16-18 mpg in my 86 Gl Automatic. This is my daily driver mostly
city driving.
It has a transplanted 87 engine with drippy heads when temp is below 40' ,
slow to shift trans cold in the morning with mobile one,spark plugs that are 5
years old( gaped and oil change every 3k), running on p205/70-14 at
32psi.A.C compressor disconnected ( 2- 40 cooling ) I do not mess with any thing
unless it is broken.
oh ya one more thing I bougt it from a friend! 4 years ago  Out of  pocket
expence since ownership is $1,100.00 . I have acess to several parted out  vans
so it helps keep the cost down.

Jim Morgan 86Gl auto "breadbox"
Westlake,Ohio