I have the ScanGauge E fitted to my car and it displays an average mpg for the current journey, along with the instantaneous mpg.
It is more accurate than the Fibometer as it is calibrated at each top up.
Most of my regular journeys tend to be over the same routes and in pretty much the same sort of traffic. About the only real variable is the weather. As a result I can check my average mpg at known points and it gives me an idea if I am doing better or worse than other journeys.
Today, my wife, who is recovering from surgery, was in the car with me. It has been a while since she has been well enough to accompany me, so most of my trips have been single crewed. What I noticed today, was at all the salient points, my average mpg was not as good as recently. It is what all the mpg forums say. Reduce the weight in the car to get the best mpg. Once she is fully recovered she can start running along behind me.