The built in MPG counter is widely inaccurate IMO.
Here's my compiled data on miles driven vs gallons filled up at the tank:
View attachment 53138
I forgot to take note of my odometer for the July 26 entry so I just added the two gallons consumed entries for July 26 and Aug 3 together.
Whenever I'm at the gas station I do the following:
1. I always do a full tank
2. I Take a photo of how many gallons of fuel I loaded up (this will be my gallons consumed since I last refueled)
3. I take a look at the odometer reading from the last time I did a full tank refuel and subtract my current odometer reading (this is my miles driven since I last refueled)
4. I calculate my true MPG by dividing miles driven by gallons fueled up
5. I snap a photo of the odometer before I leave the gas station (this will be used for my next refuel)