A constant:
Both wheels operated off the same idler arm, same motor mounting chassis, same axle.
Some background to that constant:
Eight idler arms were made ranging from .70" between the pivot and the axle, to .80" as the widest idler arm spread. The pivot and the axle mounting holes were drilled very accurately, with the drill penetrating the X on top of the idler arm, and the X on the underside; squarely aligned.
An obsessive amount of time spent determining the "perfect" distance between holes (at least as accurate as my garage level gear allows for in fabricating).
At .72" there is no hook up of the wheel. At .73" there is full hook up of the wheel. At .75" rumble is induced. At .77", the idler hits the platter, but not the capstan. So, we're literally dealing with single point hundredths of an inch between make and break, on the idler arm. Both wheels work the same within these dimensions.
By eye, I look at the profile of the idler wheel and see that it is square and parallel to the platter. As you move your line of sight up and down, from multiple angles, the wheel reveals itself evenly to the bottom edge of the profile; its "square and plumb" to the platter, and its mounted to the motor mount chassis (meaning square and plumb to the motor chassis).
So, I'm very confident that its not scrubbing, as the wheel is plumb and squarely mounted. It tracks very straight and true. The wear pattern on the platter is the same width as the wheel, no chatter marks, and no audible scrubbing via speaker playback. In fact, my Lencos need regular cleaning of the platter and show more wheel rubber transfer than this Presto.
Given the multitude of hours obsessing over these items in the build, I'm quite confident in suggesting that the mounting of the wheel isn't causing scrubbing.
Why then would there be such a difference in speeds, given the small variance between wheels?
I wish I could play this out another step, and give the real diameter measures, but I'm readying to move house, and Sarge, the spousal unit, has limited me to one deck while we're trying to sell the house, to reduce clutter for showings. The other decks, and wheels are in storage at present and not real accessible due to that (too deeply buried).
If you know a Lenco wheel, they're a knife-edge concept. The only difference is one has a rounded knife-edge, and the other has a sharp knife-edge. The differences were fairly small, but there (I might have them listed at LencoHeaven, but haven't looked).
If the theory of no difference in speed due to wheel diameter holds true, by what some suggest, why would there be a speed difference, given that both wheels have to operate from the same driveline?
If one is slightly off angle, and scrubbing (its not), then the other is as well, due to the given constants.
I'm truly curious and not trying to rile the troops. thanks for tolerating these questions.