Ford 2003 Mustang Owner's Manual page 115

Driving
fourth gear will increase your fuel economy when
you travel at cruising speeds.
Overdrive may not be appropriate for certain
terrains. If the transaxle shifts back and forth
between third and fourth gears while you are driving
hilly roads or if your vehicle requires additional
power for climbing hills, shift into D (Drive).
When to use D (Drive)
The D (Drive) position
P R N D D 1
eliminates the needless
shifting back and forth
between third and
fourth gears that your
vehicle may do when
driving on hilly terrain.
It also gives more
engine braking than
overdrive to slow your
vehicle on downgrades.
1 (First)
Use 1 (First) for when
P R N D D 1
added engine braking is
desired when
descending steep hills.
The automatic
transaxle will shift to
the proper gear to
ascend any grade
without any need to
shift to 1 (First).
Do not go faster than 61 km/h (38 mph) when in
this gear. You can upshift from 1 (First) to overdrive
at any time.
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