I have lobbied fuelly.com to
I have lobbied fuelly.com to add an "indicated MPG" field to their fuel tracker for a very long time. I think it would be a great data point to see which manufacturers are inflating (either intentionally or not) their in-vehicle MPG displays versus what the car is actually getting. I know it's all just math based on injector timing, RPM, speed, and fuel level, but the more advanced the electronics get, the more accurate the MPG should be (like maybe incorporating GPS location in the calculation).
One very important reason to track your actual mileage is that a decrease in mileage is almost always the first indication something is wrong with your vehicle. It could be a slow leak in a tire, oil getting in a cylinder head, or (like just happened to my 1 year old car) a battery going bad.
My in-car mileage didn't change from ~22, but the last 3 tanks had actual mileage of 19, 20, then 16 -- then the battery died and it wouldn't keep more than 4 volts in it.