Upvote:2
I have no idea about carpooling w.r.t. Iceland, but in the US I'm aware of a few variations:
Long distance trips: Usually arranged on sites like Craigslist, the passengers pay for gas/petrol.
HOV carpool: Specifically in the case of travel between Oakland to SF (over the Bay Bridge), I heard there are spots where drivers will pick up people for a ride. The passengers are basically helping out by allowing use of the carpool lane, which is a great benefit on the congested bridge.
Carpooling to work: The person staying farthest might always drive, or people in the carpool may take turns with driving. Passengers might contribute for fuel costs, or maybe just be there for company (and to use HOV lanes).