I feel your pain. I have a 95 gt conv and have the exact same problem. I've done the same things you have, but I also get 12-14 mpg in the city and I've managed to get as high as 23 mpg on the highway. My gears are the factory 2.73s with an automatic tranny. Oh ya... the gas tank holds exactly 15.7 gal (I ran out of gas as I rolled into the station).
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'95 GT Convertable