The number not going to move much because it's your total average.

The top line, that goes from 0 to 40 mpg, that's the indicator for instantaneous mpg. That will be moving, according how much air is needed, and thus fuel.

You may want to change the setting:

 

You are viewing "average fuel economy after reset". This number will not change quickly if you have never reset it, as it is the average of a large number of miles since last reset.

You can change the display to "average fuel economy after restart" to show what mpg you achieve for each individual drive, or "average fuel economy after fuel up" to see your tank average.

Do you have a Hybrid? Probably not with that MPG, but on the info screen of my audio system, I have the ability to "update" my trip history when I refuel to reset my mpg. It keeps a record of the dates of my fuel-ups and what mpg I averaged. It even keeps a record of the best mpg tank average ever obtained giving you a goal to beat. I'm pretty sure gas models have this feature too. Press the car icon and then go to the info screen. You should see a "trip information" icon that opens up the trip information where you can press "history" to update your mpg info.

 

I hate when someone else posts a correct reply while I am composing my answer. lol.

 

Thank you