Chevrolet 2011 Volt Owner's Manual page 429

The rubber in tires ages over time.
This also applies to the spare tire,
if the vehicle has one, even if it is
never used. Multiple conditions
including temperatures, loading
conditions, and inflation pressure
maintenance affect how fast aging
takes place. Tires will typically
need to be replaced due to wear
before they may need to be
replaced due to age. Consult the tire
manufacturer for more information
on when tires should be replaced.
Tires age when stored normally
mounted on a parked vehicle.
Park a vehicle that will be stored
for at least a month in a cool, dry,
clean area away from direct sunlight
to slow aging. This area should be
free of grease, gasoline, or other
substances that can deteriorate
rubber.
Parking for an extended period
can cause flat spots on the tires
that may result in vibrations while
driving. When storing a vehicle for
at least a month, remove the tires
or raise the vehicle to reduce the
weight from the tires.
GM has developed and matched
specific tires for the vehicle.
The original equipment tires
installed on the vehicle, when
it was new, were designed to meet
General Motors Tire Performance
Criteria Specification (TPC Spec)
system rating. When replacement
tires are needed, GM strongly
recommends buying tires with the
same TPC Spec rating. This way,
the vehicle will continue to have
tires that are designed to give the
same performance and vehicle
safety, during normal use, as the
original tires.