Fuel mileage dropped drastically overnight-help!

What area are you climate wise? In the cold areas? Winter cold has a huge effect on fuel mileage. Refiners blend gasoline for the upcoming predicted temperatures in a region. That being said winter blend gasoline has a much higher reid vapor pressure value making it much easier to vaporize and get engine to cold start in cold ambient conditions. The additives they use lower the BTU potential of the fuel meaning you need to burn more to make equivalent power simply stated. Conversely winter fuel would cause vapor lock type issues in summer or hot climates. This can make upwards of 10-15% difference in fuel mileage itself. Let alone cold weather causing longer warm ups (rich mixtures), cold stiff driveline fluids, cold tires, etc.

 

It really has nothing to do with octane, it's mostly a volatility change between seasons

 

Could be alignment. I just had mine checked with 4.5K miles on it. I have seen about a 2 mpg increase since that alignment. My first inclination (besides low MPG expectation) was the wear on my outboard part of tires. Check for wear and then consider that as a maybe?

 

Is there a way to reset the ECM without having to pull the battery terminal? My search is only finding that method.

 

I am in the camp of a stuck brake caliper or poor alignment or a combo of both. Winter gas sucks also.

 

I was getting around 22.5 on each tank. Mine has fell off to around 18.5 -19.5 with this temperature drop.