<VV> Donut Spare Tire

[Ed Dowds]

I have been told that one should not run different size tires on the drive 
end of any car, not just ones with a posi. The thinking is that different 
size tires will cause the diff to use the spider gears constantly as opposed 
to their designed function of allowing the wheels to turn at different 
speeds for turns.
A posi is designed to limit this action so the stress would be worse for 
them with different size tires.
The spider gears in a Corvair diff are not that great to begin with so 
exposing them to additional stress is not a good idea.
As was mentioned you should use a donut on the drive end ONLY if it is the 
same diameter as your regular tire.
Most owners manuals for cars that came with donut spares say to only use it 
for 50 miles. I suspect that they want you to go to the nearest town and get 
your flat fixed rather than driving with the donut. They also say to limit 
your speed to 50 mph or less.
                       Ed