You can NOT go by the computer to get an accurate gas mileage reading...they are notoriously inaccurate.
The ONLY way to get a good reading when you fill the tank up...write down how many gallons you filled used. Then divide that number into the number of miles you've driven. And ONE tank full will just tell you how you got for that one tank. Since there are many many variables that can effect gas mileage...you need several tank fulls for accuracy.
I've seen a computer reading 25....but when I actually checked it when I filled up...it was a little over 17mpg.....Sorry to burst your bubble.