Ford Vehicle Owner's Manual page 105

If you drive up or down a steep hill, your
vehicle may momentarily slow down or speed
up, even though the speed control is on. This is
normal.
NOTE: If your speed increases above your set
speed while driving in j (Overdrive)
on a downhill grade, you may want to
depress the transmission control switch
located on the shift lever to turn off
overdrive to reduce vehicle speed.
Speed control cannot reduce the vehicle
speed if it goes above your set speed
on a downhill grade. For the best fuel
economy during normal driving
conditions, leave the shift select in j
(Overdrive), or resume as soon as
practical.
Do not use the speed control in heavy
traffic or on roads that are winding,
slippery, or unpaved.
In mountainous areas, at higher elevations, or
when pulling a trailer, the speed control may
not be able to maintain the preset speed with
the transmission in overdrive.
If your vehicle comes with a 5-speed manual
transmission, you may prefer to drive in 4
(Fourth) gear, in mountainous areas or at high
altitudes, to improve speed control performance.
In order to downshift (with a manual
transmission) with the speed control operating,
you must press either the brake or clutch pedal
long enough to disengage the speed control.
Wait a second before depressing the clutch again
and making the shift so that the speed control
can fully return to the idle position.
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