I've been carpooling for about 3.5 years now, with as few as two and and many as four passengers. Have four passengers now, and it's great having to fill my gas tank only once a month.
So, with four passengers, I am using 75% less gas from a commuting perspective.
Traveling from Fuquay-Varina to RTP, off-peak commuting (work 7-4) keeps my commute to ~ 30 minutes. Add 30 minutes to either departure time, and add 10-15 minutes to the commute. So with the early departures, I am saving about 25% when compared to later departures.
I just wish I lived close enough to ride my bike.