In my sister-in-law's Ford Escort, the rear shoulder belt was just barely long enough to wrap around our Maxi-Cosi Cabrio car seat. Every once in a while, I'd have to lean on the Cabrio, pressing it into the cushion to get the buckle in.
In a few other cars, meanwhile, like DT reader David's Volvo v70XC ["Sorry, no E55 Seal Super Wagon."], the seat belt can be too short by an inch or more, and the Cabrio won't go in at all. [See above. Note: not David's actual car.]
David found a solution, although it's one that might require you to leave your affable chattiness behind when you call the Parts Department:
Thanks to the American with Disability Act, most car manufacturers offer seat belt extenders (at no cost). Volvo part #6812818 clips into the existing buckle, and rises up about five inches. With a Munchkins seat pad, the set up works great in the middle, rear seat of our v70XC.Coming next week: Using a car seat to score sweet parking spots right near the front of wherever you go.
Also, I made the mistake of telling the parts manager at our local dealership about the problem (euro car seat that does not fit euro car?!#%$#). Silly me. He would not order the part for me because they are only to be used for the front driver/passenger seats. No rear seat applications, and no car seats. Whatever, I just called another dealer, kept my mouth shut, and had the part in three days, no questions asked.