I have had my Avg mpg stuck at 27mpg a few times now, and I know that ain't right ! I have found a way to easily reset the ECU to restart the average MPG anytime I want, without taking out the battery.
The fuse is in the fuse box next to the battery under the hood. You just need to pull it out for a couple of minutes. If you have no dome lights when the fuse is out, you have the right one pulled!

See below for a pictorial diagram of where its located. It is a 7.5a fuse. Have fun!

 

When i turn off my ignition and remove the keys from the car and add gas, it automatically does that I think. Are you turning the car off when adding gas? Not sure if it matters or not.