1998 Toyota Camry Every Fall After Outside Temp Begins Drop Into Low 50 Deg

Every fall after the outside temp begins to drop into the low 50 deg range, my gas mileage goes down a couple miles per gallon. It's not yet cold enough to have to idle the car & warm it up before driving so I am unsure of what could becausing this. I am guessing that there is a sensor changing the fuel mixture or something related to a sensor that is causing this to happen. Any ideas?

Thanks,

Barry Watson
Geneva, Il.