The roundabout is supposed to fit in almost every car out there . . . while the Marathon fits front facing in most vehicles but cars with smaller back seats might have an issue with it rear-facing.
Many stores will allow you to try it out in your car for fit before purchase using a demo model - my local Babies R Us did that for us.
You can also use the www.carseatdata.com fit guide - here is a link:
http://www.carseatdata.org/search
Not sure but my friend has a roundabout and drives a Nissan Maxima. I think that car is a little bigger but not sure. I have a Honda Accord and am getting a Britax Marathon. I had a friend who drives the same car go to Babies r Us and they let her take the floor model out to try it in her car. It fit! You could go to whatever store you are planning on buying it from and ask them if you can see if it fits before purchasing it. I think most places will let you do that. Worth a shot!