If an 02 sensor is crapping out on you, that could cause bad gas mileage. I forgot whether the upstream sensors help with the air/fuel calculations or downstream ones, the ones that do air/fuel are the important ones.
Does the car otherwise run fine? Does it smell like it is running rich?
You said you run it with overdrive off? That could lower gas mileage too.
If the car was in an accident or something like that, the alignment could be off. Do your tires have enough air?
Just some thoughts...
1999 Mystique LS V6
Alpine CDA-9807, Rear deck sub, Opt. TB, K&N RU-3530/MAF adaptor, custom heatshield, FSVT 17's, SCT 4-program chip, Eibach's/ST200's, Hella Fogs...
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