Yes MT's are a pain to find pairs that have the exact same circumference. It it pretty critical on our front wheel drive with a spool. We run the 15" and only the 6and 4's. They are a much heavier tire than the std. We tried running the Goodyears (don't remember the "D" number, but that is critical too). They were always the same circumference and balanced reasonably well and did not leak. Alas, they chunked silver dollar sized rubber chunks at 300+. If I were running below that speed, I would run the Goodyears. Also there is an upper weight limit, don't remember what it is. I know Nish and Feuling run/ran Goodyears. They shaved the majority of rubber off, at least Nish shaved his off, I don't remember if Feuling shaved his or not, but I do remember that he had special Goodyears when they first came out. To answer the other question about tire pressure and runout, we adjust pressure until the circumference is the same on each axle. Now of course the big question is, what happens to the circumference at rotational speeds. Don't have a clue, but what we have been doing works so well that I'm afraid to try anything different. I do remember that we discovered the runout differences on a cement strip. When we first started, we just assumed that tires from the same mold were the same size. NOT! The car runs so straight and true now that I'm interested in more MT's 6 and 4's.
