Hello, I just bought an old M610 with bad tires. They are so old and weather checked that I cannot read the the tire size on them. I am amazed that they are not flat. How do I determine if they are 15 or 16.5? The outside diameter of the rim is 15 1/4", so I think they are 15. Can anyone verify that for me before I order some new tires online? Thank you!