Cadillac 2011 STS Owner's Manual page 401

If your vehicle has 255/45ZR18 size tires, they are
classified as low profile performance tires. These
tires are designed for very responsive driving on
wet or dry pavement. You may also notice more
road noise with low profile performance tires and
that they tend to wear faster. These performance
tires are not rated as all season tires. Winter tires
are recommended for snow or ice covered roads.
If you expect to drive on snow or ice covered roads
often, you may want to get winter tires for your vehicle.
All season tires provide good overall performance on
most surfaces but they may not offer the traction you
would like or the same level of performance as winter
tires on snow or ice covered roads.
Winter tires, in general, are designed for increased
traction on snow and ice covered roads. With winter
tires, there may be decreased dry road traction,
increased road noise, and shorter tread life. After
switching to winter tires, be alert for changes in vehicle
handling and braking.
See your dealer for details regarding winter tire
availability and proper tire selection. Also, see
If you choose to use winter tires:
Use tires of the same brand and tread type on all
four wheel positions.
Use only radial ply tires of the same size, load
range, and speed rating as the original equipment
tires.
Winter tires with the same speed rating as your original
equipment tires may not be available for H, V, W, Y, and
ZR speed rated tires. If you choose winter tires with a
lower speed rating, never exceed the tire's maximum
speed capability.
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