fat car highway milage
 

Well... 100 mile trip each way mostly highway at 62 to 66 mph

Per scan gauge ll, 32.62 mpg avg going north with 300# added and aircon.

35.21 mpg avg going south less 300# and min aircon
Maybe a bit slower speed as well and more focus,
no passenger on the return trip

ran AIT at 125 to 140

35 mpg for an '02 Grand Marquis is pretty darn good.

My dad's denny's detector does about the same, I think it does 30 MPG at 60 up a 1% grade with the headlamps on and with everything stock.

The marquis and vics are geared pretty good for Highway cruising, I believe their rear ends range from 2.73 to 3.08.

My buddies '91 vic gets 28-34 Highway.

since you were using the scangaugeII, did you by any chance check the rpm?
my Q45 turned about 1725rpm at cruise. (rear end was 2.78 and I ran tires 5.7% larger diameter)

X2. Same as my Focus with more comfy seats, almost twice the engine and probably better ride quality.