Average miles per gallon display eratic after battery change

I'm just getting used a k13 after having a k11. The average miles per gallon was showing a steady 40mpg, but I changed the battery yesterday and it went to zero and now it's changing frequently, showing 60mpg, then 24mpg and then zero when stopping the car. Is this just the ECU learning again after being reset?
If it shows zero then it's working on a running average over the last 2 minutes maybe

It might have lost the settings, there should be a way to have the average calculated over a longer period of time.

For example average mpg over the last 1 hour would show a more stable number. When you stop and idle it will come down slowly