My car is a Honda Civic, and we had the Chicco Keyfit 30 when DD was in her infant seat. It fit fine behind the passenger's seat (I have long legs, so it didn't work behind the driver's seat).
Now, she is in a convertible seat forward facing, and the Britax Roundabout fits in my car as well.
We really liked the Chicco, and will be using it again with this LO (we are pregnant again now, and due in June)