round tire wear

[Robert Deis]

Hello, all. I have a set of Khumo ecsta supra 712 (I think that's the
name..) tires on my 86 5kTQw that are wearing in a way that looks strange
to me.

The shoulders of the tires are wearing first, making them more and more
round.  The inboard and outboard shoulders wear evenly, so I don't think
its an alignment problem, and I keep my tire pressures at around 35psi so
I don't think its that either.

The rears may be wearing a bit faster than the fronts, or they may have
been rotated just before I bought the car in February, I'm not sure..

What's the cause?

-- 
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  Rob Deis                  "Let the people know beforehand what the law
  MiB3347                      is and what they are to expect."
  rdeis at io.com                              -- 18th Congress, Rec. 75