Mileage changing constantly

Just got a new battery for my 3-year-old Taurus. Now, all of a sudden, the mileage (per the digital display) varies constantly as I drive and the "miles to go" is far lower than it should be. It drops as the weather
gets colder, but not this drastically!
The mileage read-out used to change gradually throughout the year... 27.4 mpg or so in summer, working its way down to 23.6 mpg or so in the coldest part of winter. But it never fluctuated wildly over the course of one 30-mile trip, continually readjusting up and down hills. It's as if the new battery has given the display superpower and made it reactive to every little change in driving conditions.
What's going on? Did the battery installer at Mills Fleet Farm screw something up?
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Rebecca Webb wrote:

It's the new battery. There is an average mileage set into the computer from the factory called RAFE, for Running Average Fuel Economy, and disconnecting the battery wipes out the running average. As miles are put on and the new RAFE base gets larger, the fluctuations will become less & less noticeable. Do you still have the owners manual that came with the car? The explanation for the RAFE and many other things are in that book.
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Sharon Cooke wrote:

My car uses the last 800KMs to average milleage & such. I would give it a chance :)
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Phenomenally helpful! Thanks!
RW
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It is normal to get less milage in the winter. The engine doesn't run as efficently until it gets warm. And, the gasoline often has oxidizer (usually ethanol) in it, which doesn't have as much energy per gallon, so that costs about 3 or 4% (about 1 mpg) in fuel economy.
Jeff

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