Ford F series Owner's Manual page 25

Speedometer
The speedometer tells you how many miles
(kilometers) per hour your vehicle is moving.
Your vehicle contains a speedometer which
receives its input from the Anti-Lock brake
sensor. The speedometer is also used to provide
a speed signal for correct operation of the
vehicle's Powertrain Control Module (PCM),
electronic transmission, and speed control (if so
equipped). Because of this, it is very
IMPORTANT to change the speedometer's
internal conversion constant if the size of the
tires on the vehicle is changed. Changing the
conversion constant to the proper value when
the tires are changed will also ensure that the
speedometer retains the original factory-set
accuracy.
Some vehicles are equipped with a vehicle speed
(mph/km/h) limiting device that is contained
within the Powertrain Control Module (PCM).
The purpose of this device is to maintain peak
vehicle speed below a specified limit. (Vehicle
speed limits may vary based on engine
displacement.)
If you are experiencing an "engine cut-out"
condition at high speeds, it may be the result of
this limiting device. It is a normal condition and
can be avoided by reducing vehicle/engine
speed.
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