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So you got worse mileage on the Mobil fuel or the Shell fuel? I'm currently working on a 1979 1 ton with a rebuilt 350 that has had issues for quite some time now, finally just discovered that there's water in the tank, which keeps causing issues in the carb. We believe the culprit is poor fuel. Regardless, there's really no other way that water could have got into the tank besides poor fuel quality, so I think your biggest opponent now is wherever is offering worse fuel!