Tires: Tire Sizes and Interchangability ........, rim width, risk of failure

Question
Hi:  I recently purchased a single axle utility trailer with 215/65R15 Tires on it that are in pretty rough shape. The guy I bought it from gave me a couple used tires that still have lots of tread on them and look to be in pretty good shape visually. When I had a closer look at them I realized that the tires were a different size than the ones on the trailer. The newer tires are P185/65/R15. So my question is, can I replace the 215's with the 185's, i.e. will they physically fit on the existing rims and is it safe to do so. I am only hauling relatively light loads with it so load issues shouldn't be a concern.  

If you can provide me with some feedback/advice that would super.

Thanks,

Bill

Answer
Bill,

A P215/65R15 has an allowable rim width range of 6" to 7 1/2" with a load carrying capacity of 1373#.

A P185/65R15 has an allowable rim width range of 5" to 6 1/2" with a load carrying capacity of 1053#.

So if the trailer has a rim width of 7 or 7 1/2", the 185's won't fit.  Plus you are losing 640# of load carrying capacity (The max the trailer and its contents can weigh is 2106#.)  Needless to say, any reduction in load carrying capacity increases the risk of failure.