I'm not sure that you understand how the estimated range is supposed to work. I had a friend that didn't get it either (in this case, on a Ford SUV).
It keeps track of your current MPG and recent average MPG, as well as fuel level in the tank. Note that the fuel level reading is filtered to eliminate the effect of fuel sloshing around. Part of the filtering is that the PCM tells it how much fuel is being consumed. In general, fuel level is pretty accurate. MPGs, of course, change with conditions. For example, driving on the interstate, I get an average of over 23 MPG. So, with two gallons left, I get an estimated range of let's say 46 miles. Now, I get to a construction zone that is two miles long and traffic just crawls. Clearly, I'm not longer getting the 23+ MPG that I was. By the end of the two miles, my recent average MPG has gone down quite a bit. Now, instead of saying that I have 44 miles left to go, it says that I have 41 miles left to go. If the zone had been longer, it would have been even lower. When the estimate changes, the system either counts the miles down at a greater rate or a lesser rate than actual miles traveled, to quietly and smoothly adjust the display. If it didn't do this, you might run out of gas during the traffic jam with it saying you still had 15 miles to go. It has to adjust as circumstances change. For some reason, my friend couldn't understand that.
Anyway, I like the way Ford does it much better than my BMW. On the BMW, when it's estimate changes, it just suddenly changes the display. It might say 40 miles to go, then a minute later it says 30, then a minute after that it says 50. The range display does work, it just doesn't adjust smoothly.
