MPG is completely 'in the tank,' no apparent cause

Usually I quickly resolve people's low MPG complaints by adding air to all the tires and there's usually one tire that's way lower on air than all the other ones, so the MPG plummets in big ways. So that's the first step, to inflate them as much as you're comfortable with but more is way better MPG.

Of course in your case, this is just the freezing weather. Your battery ECU protects your hybrid battery in sub-freezing weather by using it way less often. Your best MPG happens at about 75-80 degrees F. at about 45mph.

 

I wonder if there is some learning that happens when you have battery issues. I had a dead battery recently twice while I was working on my car and it seemed to get worse fuel economy and slowly improve over time. Maybe it’s just me but my driving doesn’t change much

 

Disconnecting the 12v resets all the engine settings back to default, so yes... But when you're way below freezing temps there's no such thing as getting good MPG.

 

Yes. I do expect at least 5 mpg less even if I don’t idle for 10 minutes to melt ice like I did this morning.

 

I'm talking about temps cold enough that ice doesn't melt... Once you get into single digits or below zero your battery ECU is going to cut back use of your hybrid battery to less than 10% in some case and you'll be down to 20mpg no problem.

 

If you want to do anything with MPG you need data - keep a fuel log with miles, gallons, and temperatures. Do that for a couple months and then see what you're really getting.

Having a number of repairs at once and then have 12V resets here and there while relying on the MFD reading makes it tough to know if the data is any good.

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