Hi guys I know I'm reviving a year old thread but had to chime in. I just bought an 03 Crown Vic LX with the 2.73 rear gears, and just from the start with no mods or hyper mileing, I average 22 mpg.
I do know of a guy on youtube who has retrofitted a 5 speed manual tranny into his Crown Vic.
I also noticed that this car has an extremely loooong rear end. It is just asking for a kamback. I don't think it will go full on boat tail, but with the rear on this car, it could be very close. Like within a foot or so top to bottom. As was stated earlier, the front is bad aerodynamically. Lots of big, gaping holes.
Something else to consider is Netgain Motors. They mainly sell parts for conversions to all electric, but also sell the EMIS system.
EMIS - Engine Motor Interface System
It is made for cars and trucks with automatic transmissions built after 1996. It replaces your stock driveshaft with a driveshaft that has an electric motor built into it. It uses a 48 or 72 volt system and connects to the cars computer to strategically apply power as needed for better fuel economy.