Tires: Can I put these tires on my minivan?, r15 tires, sized tires

Question
I've got a set of 195/65 R15 snow tires from a 2006 mazda 3
that I no longer have.  I was wondering if I could put them
on my 2007 Toyota Sienna.  (it currently has 215/65 R16).  
The curb weight on the sienna is 1895kg.  The max load on
the 195/65 R15 tires is 615kg @ 51psi.  Thus they should be
able to carry the minivan plus an extra 565kg or 1245lbs
(that's assuming a 50:50 weight distribution).

Is this completely unsafe or a really bad idea?

Thanks!

Answer
Matt,

In engineering, the basic principle is to design the product (tires) so that it is capable of more than its design intent (the load / speed / etc.), and then utilize that product (on a car) in a capacity that is less than its design intent (less load / speed / etc.).

So car manufacturers specify tires that have more load carrying capacity than the minimum.  Nevertheless, tire failures do occur and sometimes load is a factor - which is why tires with more load carrying capacity (and tires with higher speed ratings) are always less risky to use.  If the only risk were that the occupants of the vehicle were inconvenienced by a tire failure, well, that wouldn't be good, but since the possibility exists that a tire failure could result in a tragic event - well - that makes undersizing tires bad!

But there are other reasons car manufacturers use larger tires than the minimum.  One is that handling is much more predictable, and another is that a larger tire is more pliable and gives a better ride.

I recommend you sell the old tires and purchase properly sized tires.  That would be the safest choice.