Is the speedometer in error all of the time? Need to know how much is it off.
Set the cruise control with traffic. Record the what the speedometer says. Time how long it takes to go a mile on the expressway. The mile markers are perfect for this.
Now you get to have a real world example of those math problems everyone hated in school.
It would also be nice to know if the prior owner has swapped transmissions. My gut tells me this is the case.
If this were a rear end ratio change, say for example 4.10's used instead of 3.27's, the speedometer would read higher than true speed.
If asked to handicap this thread, I would say the prior owner used a transmission from a 04 MY which has a different number of teeth on the OSS reluctor wheel. If true, this can be corrected with a tune.
But what we have to know if an older transmission was used.
Additional helpful information is to know what the PCM "thinks" the car's speed is. This will require an ODB2 scanner that is capable of accessing operational data.
Pictures af the transmission tail shaft would be a great place to start.
As for the gas gauge, my bet is the sending unit was damaged during a fuel pump upgrade/repair. Are you willing/able to drop the fuel tank if need be?
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.