Tires: changing tire size, passenger car tire, tire failure

Question
I have a trailer with 205/75D/15 tires on it and I also have a single tire from a friend which is 215/70/15. Could I use this  on my trailer without any problems.

Answer
Tim,

Does the trailer tire have the letters"ST" in front of or behind the numbers?  If so, then you can not use a passenger car tire on the trailer.

Even if the letters are not there, what you are proposing is going down in load carrying capacity, which increases the risk of a load related tire failure.

Basically, the answer is, No.