.
Jack the car up and check if a brake may be dragging.
If you have a Ultragauge, check and see if it is coming out of open loop enrichment. The loop in the top right corner will be open when cold and close after a few miles of warming up. Using a ultra or scangauge will provide a lot of other info that may help identify the problem.
Other than that, I would keep taking it back to the dealer. Document
everything. If the problem persists, tell them your going to get a attorney. In light of the EPA cracking down on manufactures false claims of mpg, they may offer some sort of compensation.
Remember, a squeaky wheel gets the oil...
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