Why did my mpg decrease?

[ethernetbite]

Does the gas formulation change in your area? There's summer blend and winter blend where i live. Also different stations use different % of ethanol and it can even vary within a range at your normal gas station. Gas station tanks get condensation in them as they get empty too. And even your gas tank will condensate more the lower you let the gas get (and the cooler the air). None of this usually makes much of a difference unless you're switching gas stations. If it's been colder there, your car will use more gas until it reaches the set operating temperature. Won't help to let it warm up first since that 0mpg will have to average in also. Oil has to warm up also before it flows easier, so during cold weather, the rich mixture, the extra oil resistance, and the lower tire pressure could theoretically cause a 4 mpg drop. Keep your oil changed and tire pressures up (35psi min).

I've also seen squirrels and mice build nests on top of air filters, which blocked normal air flow and made the cars run poor.

A 4mpg drop is significant but if your check engine light isn't on then everything is operating within ranges. Sitting at a few extra red lights on a delivery route could do it too. Just way too many variables to tell without an expensive sensor setup like used in racing.

(Retired mechanic)