Ford Mustang Owner's Manual page 23

Speedometer
The speedometer (GT similar)
The speedometer tells you how many miles
(kilometers) per hour your vehicle is moving.
Odometer
The odometer tells you the total number of miles
(kilometers) your vehicle has been driven.
Trip Odometer
If you want to track your mileage up to 999.9
miles (kilometers), use the trip odometer. Simply
set the trip odometer to zero by pressing the
reset control firmly when beginning the distance
you wish to measure.
Since the trip odometer displays distance
independent of the odometer it will not always
advance to the next mile (kilometer) at the same
time as the odometer.
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