Anything after 93 was a digital speedo and easy enough to change with a speedcal box or a tune. Easy way to save gas is slow the hell down. I do 70mph with my 3.73s in my 98GT and routinely get over 20mpg hand calculated.
And being a 2004 you are running rich right from the factory. The computer is tuned for 19s but those run at 90% duty cycle on the PI 2V so in the last few years they went to 21s and never changed the tune. Get a good tune and it'll lean out a little and save you some gas that way.
However, you have a sports car, if you are THAT concerend about fuel economy get a beater to drive to work. A mid 90s Civic is $1k and gets near 40mpg.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.