When to create a new content type?
It's a really open question, but I'll try to help you a bit.

Basic example is : I have to create "vehicules", some "cars" and some "motos". Should I create one content type (CT) with one field "type" (with "cars" and "motos" as available values") or 2 CTs "cars" and "motos".

Here is the points I look into before choosing:


fields: if the fields are the same between content types, just create one CT, if they're really different, then create 2 CTs.
display: if the display is the same, then chose 1 CT, if not, create 2 CTs.
views: will I make list of both CTs, or will they always be separated?
cross content: is it possible that a node is part of both CTs? For example, is a "can-am spyder" a moto, a car or both of them? If you have cross content, then I would suggest to have only one CT, which a multiple select for the "Type" attribute. So people searching for cars or for motos will find the spyder anyway.
how "smart"/involved are you Back-Office users? Usually when you choose to keep only one content type, you end up having a lot of extra fields to configure the theming, the little differences between your different types, etc. So the edit form can be really long and confusing. If you're the one configuring everything, that's fine. But if you need to explain the form to a lot of editors/contributors, this can be painful.


After counting the pro and cons, you can make your decision.
Usually, I give 2 points for "fields" and "display", one point for "views", "cross content" and BO users... Make the sum and decide :)