In the garage, I have recorded the MPG displayed on the factory gauge for almost every fillup. Leaving out the two I missed, there are 178 fills with these data. Since I'm taking a statistics course this semester, I thought it would be fun to run a simple analysis. Here's what I found:

Using the quartile method to identify outliers, I removed two data: the two times when the factory display underestimated calculated MPG. The remaining 176 fills chart nicely into a normal curve.

Median = 3.0 MPG difference

Mean = 3.04 MPG difference

Percent difference shows an even clearer normal curve, and again median and mean are very close to each other.

Median = 5.58% difference

Mean = 5.563% difference