Miles to empty inaccurate after Aux battery replaced

Run it for a couple tanks and see if it will go back to normal. When you top off a Prius when it's not low, it will always have this problem

 

The DTE display uses recent MPG history to compute a figure. While replacing the 12V battery, the power loss likely corrupted the history memory, or even erased it back to the factory default.

Don't worry about it, it will automatically get "fixed" with miles driven and as it builds up the history memory since the electrical power loss. This display is not something you should be putting all that much faith in anyway.

No, it won't. By then, it will have more new mpg history available, and the 'fuel remaining' portion of the distance-remaining equation should be accurate. It isn't like the MTE display just decrements its counter at the same rate the odometer increments. It is continually making a new computation, and can fall much faster than the odometer increases.

For an interesting contrast, my Subaru bases its DTE on a much shorter history, and the display number is allowed to increase. I've even seen times (under special conditions of high MPG following a long slog of low MPG) when its DTE actually increases faster than the odometer. But my Prius is different, its DTE display is allowed to increase only when it detects added fuel. The rest of the time, some internal rule forces it to decrease at least 1 mile for every 2 miles driven, even on very long downhill glides that burn no fuel.

The one time I added a single gallon (expensive fuel, for margin to insure reaching reasonably priced fuel farther away), it was not enough to reset it and show up as a boosted DTE. So DTE continued its monotonic march down to 0, though at the slow forced rate of 1 mile decremented for each 2 miles added to the odometer.

 

Not a good idea to trust either.
If you have an average MPG you trust, figure it's a 10 gallon tank, (I KNOW, it's 11.6) at est. 48.5mpg,
485 miles a tank. That would give you a "safe" range so you are less likely to run out of fuel.
Might be a good idea to fill up when the gauge gets to 1/2 or 1/4.
My average fill up is usually 1 bar above or below half.
Traveling, you never know when you might get stuck on the interstate due to a wreck. A half a tank
of fuel in a Prius can keep the car running for a long time!

 

Yeah the Distance To Empty gauge is basically parroting the gas gauge, in a round-about-and-vague way. Go with the gas gauge, be conservative in estimating your range, fugedabout DTE.