> >- a difference of greater than ~5 RPM across the coupling will initiate lock
> >up process.  Such a difference occurs with 3mm tread wear diff. F to R on
> >175/70/13 tires at 160kph. check those tires/pressures folks.
> >

I thought this was a Friday-thing.

I would certainly not fear for my VC as much as for my engine it I tried to
drive a Syncro Vanagon at 160kph, while it was fitted out with 175/70x13
tires!  The poor old tach would be really up there!  I wonder what the 
handling would be like?  At 100 miles an hour in a Vanagon with such tires,
I'd be concerned about my mental health, since this would constitute
attempted suicide in most folk's opinions.

Since this is obviously not about Vanagon/Transporter units, I think you can
safely discount any "3mm" differences.  You are looking at a percentage
difference to get rate differential, and a speed.  What they are saying is
that at 100 miles an hour, these bitty-tires are spinning really fast, and
a 5rpm difference that triggers whatever syncro unit they are talking about
*can* actually occur.

My syncro has 215/75x15's on it, which are *way* bigger, and I don't often
feel compelled to run this monster down the road at 100mph.  I agree that
it is good to run the same size tires, but this thread is getting really
paranoid about nits.

I don't know if the 5 rpm difference before locking applies to the Vanagon,
although this does not seem unreasonable.  But do the math first, and think
about the figures before you start getting upset at 3mm diameter differences.
I would suspect that the VAG guys quoted were talking about either the newer
Audi Quattro's or Golf Syncros (the quattro system, I hear, was dropped in
favor of the syncro system).  I also would expect you to feel this lockup in
a any car where it occured.  Typically on a Vanagon, the front wheels would
slip, and you'd momentarily feel shuddering as the syncro unit locked and
unlocked (bear in mind that it cannot stay locked, since it depends on rate
differences to lock, and once locked there are no rate differences).  On a
sports cars going 100 miles an hour, this could be disasterous.  For the larger
tires on a Vanagon, the same thing would require substantially more speed.
The locking was considered desireable in an off-road and poor traction
situation, but this behaviour is not desired at high speed -- all that is
desired is that power be transferred.  The comment on improved braking would
be a notice that there are benefits to syncro-locking even at speed -- it
provides an active brake anti-locking balancing system.  Conventional
anit-locking stops braking on a locked wheel, while the syncro would
assure that all the wheels braked more-or-less evenly.  I did not read in
this quote that it was prompted by syncro-unit failures, and I doubt that these
occur commonly even in this situation -- I expect you would stop driving as
soon as the syncro started it's locking at speed.  You can feel it lock if
you drive in a tight circle -- it is not something you'd miss on the freeway
I'd wager.

Put on the same size tires, keep them inflated to the specs, rotate them. 

Worry about meaningful things, like your kids or finals or which beer is best.

malcolm