Actually most modern cars don't offer much in terms of a fuel economy meter.
I remember when BMWs had the "economy" gauge in the instrument panel.
It was purely an engine vacuum gauge which would just go up and down so wildly, and provide no accurate measure whatsoever. It was always fun though to see it bobbing up and down so wildly though. I believe they actually fitted that useless superfluous addition for around ten years. It's gone now of course.
These days the manufacturers tend to offer fuel economy results after the fact, which means that they are far more accurate, albeit too late, as the driver has already spent the fuel by the time they get the results.
There are some more useful examples perhaps in a few of the hybrid and electric cars which offer realtime information based on a host of inputs and equations.
It would be nice to see this embedded into more modern vehicles though, as its a very useful addition as you say.
Still, the issue again with even the hybrid and electric models which offer a live feed, is that they are only so accurate. As it was mentioned earlier in the thread, using quite a lot of power output to get up to cruising speed more quickly, actually can result in greater fuel economy. Even the most modern of fuel economy meters don't work in this scenario, as they aren't predictive. This means that they would all give a poor economy reading while you were performing this action, and many drivers would think to back off the throttle as a result. It would only be after the fact, and on the five minute economy scale, that the saving would be shown, but then again, the driver might not understand after a five minute delay in the economy result, where the resulting saving came from.
So again, in this sense, even a live fuel economy gauge would be as useless as the old BMW engine vacuum gauge, which was phased out.