I had the EXACT same problem

. I smelt gas, gas came out from underneath, and i got the worst gas mileage. My problem turned out to be the rubber gasket that connects the pipe from the gas lid to the tank. It had to be replaced. It wasnt a big dollar thing either, just had a lil shop do it, they didnt even have to pull the tank out although i needed to get rid of most of the gas first. I hope u have the same problem cuz it can be fixed in 2 hrs at the most.
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94' Stang GT Convertible Cobra upper&lower, Mac cold air w/K&N, BBK equal length shorties, BBK offroad H-pipe, American Thunder cat-back, BBK T/B 65mm, 24lb injectors, Pro-77mm MAM, pulleys, aluminum driveshaft, 3.55s 14.4@100. (Sad, i know. i have stock heads)