i'm at zero.
i live in a highrise downtown condo (no lawn) and don't own any motorized vehicles (just 3 bikes, 1 kayak, and a CTA pass).
however, i am a member of a car-sharing service, so if you want to get technical (and who doesn't?), i guess i'd have to add up all of the cylinders in their fleet of cars and then divide by the thousands of members in the organization to get the fractional "ownership" stake i have in one of those cylinders.
the most recent I-GO car sharing stats i could find are from 2009, but as they add cars and members, the math probably doesn't change radically anyway:
185 cars x 4 cylinders (all 4 cylinder hybrids) = 740 cylinders
740 cylinders / 12,000 members = 0.06167 cylinders per member.
that's still pretty damn close to zero.