rule of thumb = 5 speed has better highway where as the auto has better town
Now on a tank of gas i see anywhere from 22-26 and that is with a little spirited driving put in.
Couple things to look at and do.
1) CLEAN THE MAF SENSOR WITH MAF SPRAY "Can not be over stated... picked up power and about 2 mpg when i did mine"
2) EGR Valve might be old , may need new one
3) Change the PCV valve... How old is your pcv valve? suppose to be changed every 20k i think
4) Spark plugs gapped to .054 or .052 "one or the other"
5) How old are your wires? if over 60k up to 100k you might want to change them "helped me out a little"
6) Maybe its something in your tune that is killing economy? did you update or change the fuel maps?
7) As stated above , could be clogged injectors, dirty fuel filter "ps. if you run your tank dry it sucks a lot of shit into the filter and if you run it really low it does it also", exaust leak "highly plausible with long tubes", I see that you go 60/65 but how long does it take you to get there, and how long do you stay at 60/65 "because if you just drive short distances the car isn't going to get good milage", another note on a tune, running lean gives you better gas milage than rich... , also do you use cruise control? i could help if your in flat lands or if its hilly then don't use it as much b/c hills cause milage to go down, what psi do you run your tires? front and rear? could you have brake binding? When is the last time you added a bottle of fuel system cleaner?
As you can see this list will keep going and going. But its coming from somewhere. Just check these items off in your head and fix the ones that are easy and then start working your way threw. I would also add a bottle of fuel system clean to every tank of gas for a while and see if that helps.