Michael Hamm writes:
Hm. Good point. Now I've got to check whether the lap portion of the
seatbelt also extends from the seat. (I suspect it does, as the car seat
seems rather loose.)

Shoulder belts only have one spring-loaded pulley, at the upper mounting
point of the belt. The lower end of the belt is attached directly to
the anchor in the seat base, which is immobile. If you clamp the upper
end of the belt, either using a built-in clip or an external clip, so
that the belt from the lower anchor to the seatbelt latch and then up to
the clip is all fixed in place, and you pull this tight, then it doesn't
make any difference whether you can pull the upper, slack part of the
seatbelt out of the spring-loaded pulley.

The key word is *tight*; you have to get the lower part of the belt,
that's actually holding the carseat in place, to be tight. Most people
don't do this properly, and thus most carseats aren't safely installed.
If you aren't sure you know how to do this (which is the impression I'm
getting) you can usually get the local police to show you how to install
it properly.

David desJardins