Long before before $4 a gallon became the national average, many newer cars already included dashboard instruments that gauged fuel consumption. But it's the older gas guzzlers that arguably needed them more, to show how much money was being wasted models manufactured in the pre-hybrid era.
That's why the K.A.T. Matrix 3-Axis accelerometer may come in handy as a reminder of how much careless driving costs at the pump, depending on one's performance in horesepower, G-forces, quarter-mile speeds, and 0-60 clock time. Or, as Dvice says, it can be a badge of honor for those who simply don't care and want to display that fact for all the world to see on the windshield.
Either way it's apparently easy to install, requiring just 3 AA batteries for power. But remember, it has an "error compensation feature" in case you think of cheating with the stats.