Yep, it's
never been
discussed before. 
Anyway, it's sometimes hard to say. Even if you carpool, if you want to be literal about the MPG, until you pick up the first passenger, or after you've dropped them off, you're still in a single vehicle, doing the math seems like more work and requires a well calibrated Scanguage to properly do it, espeically if you're got 3 or more people.
With familes it's a bit easier, but then what about people who are just coming along for no real reason?
Personally, I carpool every week, and I'm the driver when we carpool because my vehicle is the more efficient one, and have done so for years.