Can we take a half step back... Why are we thinking that cooler weather would decrease fuel economy. What about the theory about cooler air is more dense and therefore creates higher cylinder pressure when heated. If anything, cooler air should increase economy. One thing I have noticed recently in western NY is most every refueling station has posted an ethanol content label on the pumps. If you now have ethanol supplemented fuel, it will immediately reduce economy.

 

cooler temperatures makes fluids thicker until they warm up because of the vehicle moving around. Not only motor oil, but transmission fluid and differential fluid. differential fluid is very thick. Thicker fluid makes it harder for the engine, less fuel economy. Since he's in CA...don't really have to worry about that much.

 

Correct me if I am wrong, but does the ECU calculate MPG based on how much the injectors are putting out at any giving time? The ECU tells the injector to open for what it needs to be(time), and knowing flow rates of the injector the ECU generates a MPG number to be displayed on the dash. I know too that ethanol mixed in the gas will change the fuel density and the specific BTU content per gallon, does the ECU sense the O2 amounts in the fuel for it's fuel delivery map? (like the flex fuel cars).
Rob

 

Cooler weather = less efficiency...winter blend fuels have less energy. How bad is bad fuel economy? down 5-10%? If so, I'd say that's normal