B.H.
I have a ford focus hatchback 2door car. Pretty small...
I fit the Britax Regent in my backseat just fine actually fits better than the Marathon does. It does take up one whole seat and some of the middle seat so if you need other passengers back there it is wide. I have a only child though so not a problem for me.
The Sunshine Kids Radian 80 is also a great option and not as wide. I have seen a family with sextuplets use that seat in a van and everyone fits.