My '98 GT automatic has 3.73 gears and my mileage is great. I get 20 mpg in town and anywhere from 24-27 mpg on the highway. Rpm's at 70mph are less than 2500. She actually gets better mileage when I lock the cruise control at 80mph! Gears are the first thing you should do.
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SD Wheeler
M/M #30 '98 GT
MV Performance
Blade Runner Racing
"The Game"