BAD GAS MILEAGE SINCE OIL CHANGE!!!!!?

I have a 1999 SAAB 9/3 and until a week ago the gas mileage on it was consistently 20.1 mpg using the information display (SID) . However, after I took it to firestone for a oil change the gas mileage dramatically dropped to 12.3 mpg! After a few days of seeing a consistent 12.3, I went to talk to the mechanic who... show more I have a 1999 SAAB 9/3 and until a week ago the gas mileage on it was consistently 20.1 mpg using the information display (SID) . However, after I took it to firestone for a oil change the gas mileage dramatically dropped to 12.3 mpg! After a few days of seeing a consistent 12.3, I went to talk to the mechanic who changed the oil and he said that he didn't do anything and oil changes have no affect towards gas mileage.

I'm sure what I should do next..I'm really concerned.
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