Ford Thunderbird Owner's Manual page 164

Consider using one of the lower gears.
To avoid skidding and losing control on
slippery roads, do not downshift into 1
(First) when you are moving faster than
20 mph (30 km/h).
Do not use the speed control in heavy
traffic or on roads that are winding,
slippery, or unpaved.
If your vehicle gets stuck (for example, in mud
or snow), you may try to "rock" it out of the
spot. Shift, in a steady rhythm, between forward
and reverse gears. At the same time, press
lightly on the accelerator.
Do not spin the wheels at over 35 mph
(55 km/h). The tires may fail and injure a
passenger or bystander.
By moving the vehicle backward and forward,
you may gain enough momentum to move out
of the spot. Do not rock the vehicle for more
than a few minutes. This may overheat the
engine, damage the transmission, or damage the
tires. If you are still stuck after a minute or two
of rocking, call for a tow truck.
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