The “miles to empty” estimator works by factoring in what kind of mileage you have been getting lately. If you replaced the battery, and then drove directly to a nearby gas station to fill up, then the only information the computer has to go on is “mileage is absolutely terrible” because short trips where the car barely even warms up get much worse mileage than normal trips do.
For future reference, the only remotely accurate way for you to figure out what kind of mileage your car is getting is to divide the number of miles you drove since the last fillup by the number of gallons you put in on this fillup. So, 300 miles, 10 gallons, you’re getting 30mpg.
Any other method (at least, any that you can get your hands on) of determining mileage is too inaccurate to be of any use whatsoever.