No Gas Mileage...HELP!!!!!!

[screaming_pas$enger88]

i know, im just full of questions today.
when i bought my car, it had 4 cyl. guages in it, and it still does. i didnt think anything of it, i just bought a monster tach and that was that. but now that i think about it, i only get about 100 miles to the tank according to my fuel guage/odometer. so what im wondering is if this car was a prior 4 banger would it maybe have a smaller gas tank?? or if it was an original 5.0 and someone just put 4 cyl. guages in it, would the gas guage read differently, maybe not understand the sending unit or something????? i know even the biggest piece of crap stang should get more than 100 miles to the tank......

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AIM= OneQuickStang