bad gas mileage after oil change at dealer??? and they never changed the air filter...
if you know for sure milage is worse,i would tack back to dealor and complain.
It's possible that they did not use the recommended weight oil -- for example, many import cars call for a 5W30, if they used a heavier weight oil, you could see a decrease in gas mileage. Other than that, it sounds like bad coincidence, an oil change really should have no noticeable effect on gas mileage.
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