[mdoyle]

I had a similar dilemma when I had a virgin spare and two used take-offs. I was considering buying one new tire and then using the other three I had to make a newer set.

The main concern was the difference in wear between two of the four tires. I couldn't find anywhere that stated an acceptable wear differential, so considering the big picture I replaced all four with new Toyo M/T's.

For certain different tire diameters will cause the differentials to work harder, move up your time table on new tires and get them now instead of potentially more expensive repairs later.