In order to get horrible gas mileage, you have to be running rich unless your car has the mods to justify running rich. Not running rich but getting bad mileage means the car is getting more air to be able to dump in more fuel, and also has the ability to burn all of that off.. And hey, your mileage can't be as terrible as mine... With a cam and a 4.11 rear, my best guesstimate is 8 mpg city. 
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'91 Mustang GT 5.0 AOD Daily Driver
Current stats: NEW 302 short block, E-303 cam (@.050, 220/220, .498/.498, 110 degrees), 1.6 roller rockers, Performer RPM heads (aluminum, 1.9/1.6, 60cc chambers), Cobra intake, Granatelli 75mm MAF, Billet AFPR, Equal-Length Shorty Headers, Off-Road X Pipe, Flowmasters, 4.10 POSI
13.73 at 103.8, ~2.2 60'
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