<VV> Doughnut Spare Tire

[Jim Simpson]

There are actually three varieties of limited use spare tires.  One is the
small, so-called "space saver" spare that most of you are talking about.  A
variant on that is the inflatable spare that is even smaller and requires a
can of compressed gas (propane?) to inflate.  The third variety is the full
sized, light weight tire.  (I have one of these in my Audi wagon -- it's
full sized and mounted on an alloy rim and looks almost identical to the
original wheel/tires on the car with the exception of prominently molded
warnings about speed and distance limits.)  Also note that most of these
limited use spares require different inflation pressures than the standard
tires.

All three have prominent warnings regarding speed and mileage and are not
intended for extended highway use.  (Generally 50 mph and 50 miles.)
They're just intended to get you somewhere where the regular tire can be
repaired or replaced.  (I too remember reading a test to see how these
space savers handled and braked -- surprisingly well!)  But never-the-less,
they are not a regular road tire.

Cars with limited slip differentials or cars will all-wheel drive need a
spare tire that matches the rolling diameter of the regular tires.  Hence
my Audi, with its Quattro all-wheel drive, uses a full size, but limited
use, spare.  (I assume it saves a small amount of weight and a couple
dollars in manufacturing...)

If you have a Corvair with positrac (my Corsa turbo does), then you need to
make sure you match the rear tire diameters.  You can get by with a smaller
diameter space saver spare, but then you might end up in the situation
where you would need to swap a front tire to the rear and put the space
saver on the front.  And then you'd have to inflate the former front tire
to the proper rear tire pressure.

All said, I've only experienced two catastrophic tire failures in 50+ years
of driving.  One was a tire sidewall failure due to a manufacturing
defect.  That one I noticed before it actually popped -- it blew out after
I'd put the spare on.  The second was a tire that my wife ran into a "V"
shaped pothole -- wide at the end the tire rolled into and then the pothole
sides converged to rip the sidewall out.  Everything else has been picking
up a nail or screw and then having a leak that needed to be patched.

So, as they say, "your mileage may vary".

Jim Simpson
Group Corvair