Honda Civic 2012 Gas Mileage

Earlier today I noticed my MPG on my car had gone up about .3 mpg in a matter of less than an hour which was weird because I didn't recall seeing it go up, just seeing it at 28.5 then an hour later at 28.8. After finagling with it I realized something was definitely off. At the bottom of my screen instead of the miles on my car there was an uppercase A and a random number. I attempted to get it back to my normal settings and after I did that it was still not right. Now my mpg changes frequently and rapidly, almost every few seconds. It will go from 31-21 in a matter of seconds. I would greatly appreciate if someone could help me get my car back to normal. Again I have a 2012 honda civic

Sounds like you changed display from (Average) miles per gallon, which will change slowly between tank fill up. To (Current) miles per gallon, which will constantly change to show what you're getting at that point in time, it will be low when accelerating and very high when coasting.
All you need to do is change display back to Average mpg.