We have the Britax Diplomat. In our car the tether has to hook up to the frame of the front passenger seat when it's rear facing, so the passenger seat can't easily be moved forward and back. Other than that it's a pretty small car seat. It was one of the most narrow ones I could find. That was the main reason I got it, just in case I ever have to squeeze three car seats into the back seat of my car. I know some people don't consider the Diplomat since it only goes to 40 pounds, but Allison is small enough that wasn't really a concern for us.
Re: yet another carseat post - small car?
Katie: 1/16/08 2lbs. 15oz.
Abby & Emily: 12/31/10 6lbs. 2oz. & 5lbs. 7oz.
We have the Evenflo Triumph Advance Premier (which is a big seat) and it fits fine in our Corolla.