I have found tires as large as 235/60R15 to be about 26.5" in diameter. If I am not mistaken, that is the approximate size of my 295/50R15. I have also measured 14" tires for this size range and come up with 205/70R14. In a 15" tire, I would guess at a 205/70 or 75R15 for the 26.5" diameter. Sites like Tirerack can help with this kind of sizing requirement, and you will be able to get the tire you need.