On my old cars I'd suspect engine/transmission but a modern vehicle would throw a Check Engine error code for misfires, wouldn't it?Usually hovers around 15.5. It’s as if you can watch the gas gauge needle going down!
I wonder if one or more brakes are dragging. You'd notice deceleration when you coast; brake discs too hot to touch.
Did you check the tire pressure? I expect a tire would have to be visibly low to give MPG that bad.
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