[ACS]

AT my last oil change I told my service advisor my MPG had dropped by about 2 (with no change in my driving conditions). He said it was due to changes in the barometric pressure. This sounded odd to me since it was relatively sudden and I have gone through summers with the car.
Can anyone tell me if my mpg is off (I will check with my next gas fill-up) or if there is a problem that needs to be corrected?
thank you