If I may add an observation:
Lifetime MPG may or may not be useful beyond your own curiosity. Why, you may ask?
Have you ever bowled in a bowling league or a golf league? Remember how easy it is to improve (or damage) your average at the beginning of the season? And how difficult it is to make it change much by the end of the season?
You will notice the same thing if you're keeping track of your car's lifetime MPG. After 10 or 20 tanks or so, it will be difficult or even nearly impossible to budge that lifetime MPG one tank at a time. Only by modifying your driving over the NEXT 10 or 20 tanks, could you stand a chance to modify the LMPG.
Likewise, when your car is 4 years old, it may take another 3, 4, or more years for you to notice a marked difference in your LMPG, even if you have turned over a new leaf, and are driving much more conservatively.
Hence, my opinion that over-reliance on your LMPG might be not as useful as you might think, and it may even set you up for frustration over time.
On the other hand, I believe that tracking my MPG tank by tank, week by week, or even month-by-month is far more useful. If I have the car long enough, I'd be able to compare June '05 or June '06 with all prior June MPGs, to see my summertime driving trends. I think this would be much more useful to me in terms of behavior modification and self-improvement.
Of course, I'll probably use a spreadsheet or the mileage database on greenhybrid.com, so my LMPG will probably be tracked anyhow...but it will become less and less useful over time, until a point comes at which I'll just ignore it entirely, because it won't change a lot from tank to tank.
Did I make any sense at all?