It will just keep averaging out overall. If you want an average per fill up, reset it.
On a side note, for the past 3 years when filling up the miles to empty has fluctuated from around 380-415 miles at best. Last week I fill up and the it reads 545 to empty. It stayed that way and didn't drop significantly ove the past 2 weeks. Even my son noticed the large increase from the normal 380-415 fill up range to the 545 mile. Anyone got a clue?
If you had a lot of highway miles on the last tank of gas before you filled up it will change the miles to empty.