With the running 90-day average, especially right after winter, the skewed mpg figure makes a driver's results look like they're worsening. In my case, my fuel efficiency has actually increased every year, but I'm running below my overall average because of seasonal differences.
I've even been able to quantify the gallons and dollars saved over my first year with this car. In the last three years it amounts to enough to more than pay for the ScanGauge II.
In addition, the chart doesn't show the latest figures at all.