Ford Mustang Owner's Manual page 168

Upshifting
Most of the time, you should upshift at the
following speeds:
Shift Speeds for 3.8L Engine Models
Shift Speeds for 4.6L Engine Models
* In these gears, you may sometimes shift at
lower speeds than those we suggest to improve
fuel economy.
Sometimes, you will upshift at higher speeds.
For example, you may want to stay in a lower
gear to gain more power for passing or for
climbing a hill. However, you must stay within
the allowable tachometer range when you
upshift or downshift. See Tachometer in the
Index.
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