hi tom - i noticed that you said that the 4.6 has plenty of horsepower, that's what i thought, but found out different.
this is a 1993 lincoln towncar, with tow package and the 210 hp engine. without towing anything, on the open highway @ 70, it will average 27 mpg, even with 150,000 miles on the engine.
i bought a 5 X 10 enclosed doolittle trailer and used the lincoln to drag it to vegas and back for a furniture show. it was over 4500 miles round trip. changed fluids and filter before i started out. going west my gas mileage dropped from 27 to 10.4 mpg and coming back it went up slightly to 11.3 the car had hardly any pickup through the mountains, but yet the trailer seemed to roll free enough and would gain speed quite easily coming down the mountains. i've hauled this trailer with my old pickup that only gets 11.5 mpg and has a 400 under the hood and it might drop it to 11.3 mpg dragging the same trailer.
so, why the big dip in gas mileage? when i pulled the trailer, the lincoln only had about 100,000 miles on it. it driven 50,000 miles since and have had no transmission problems and the transmission nor engine has never been rebuilt. the gas mileage is consistent in the high 20's.
so, this is what i had concluded. lincoln designed the engine and car to function at it's best carrying itself. as soon as you pack on the extra weight, it starts laboring and the gas mileage heads south. it does it every time i carry much over 1000lbs in it, whether it's people or product. crown vics probably will behave in a similar fashion. just my .02 cents worth.
~ curtis
knowledge is power - growing old is mandatory - growing wise is optional