Tires: Difference in Tires, trailer manufacturers, bias

Question
Hello,

I have a trailer with st205/75d14 tires on it. I am trying to get a spare for it. I found a spare for sell locally but it is a st205/75r14. What is the difference in the "D" and the "R"? And would it be ok for a spare or no?

Thank you

Answer
Josh,

D is "Diagonal" - which means "Bias" and R is Radial.  That would be OK for a spare, but I would keep the set on the ground the same.  Radials deflect a different amount and this could cause overloading of a given tire.  Trailer manufacturers are noted for using tires that are too small.