Like Sal, I'm using the standard Cobra size. They do wander all over the road like a Kennedy crossing the Chappaquiddick, especially when cold. But, on the track they are just fine. And yes, they do lose air, though mine don't seem to go as quickly as Sal's. I have not had the ballance problem that Sal describes, but I have heard that if they sit, they will go out of round. Mine have never sat
since I've owned the car. 3800 miles so far, and no problems.
Since I like the "look" so much, and do track my car, I'm willing to put up with the wandering on the street.