The thing I love about the English language is that it's chock-full of exceptions to the rules
If you want to pluralize a word, you add an 's' (car -> cars) -- except for when you have to add 'es' instead (bus -> buses). So it seems logical to me that there might be a similar exception to the rules of making posessives -- if the cars all have flat tires, it's "the cars' flat tires." If one bus has a flat tire, its "the bus's flat tire."
Plus I write by ear, and without the extra s at the end it just doesn't "sound" right to me
However, I'm perfectly happy to let others choose a different scheme, since apparently several of them are correct.
As someone said, the main thing here is consistency -- pick one, and stick with it!
PJ
