Ford 2003 Mustang Owner's Manual page 172

Maintenance and care
Use a known quality gasoline, preferably a
national brand.
Use the same side of the same pump and have
the vehicle facing the same direction each time
you fill up.
Have the vehicle loading and distribution the
same every time.
Your results will be most accurate if your filling
method is consistent.
Calculating fuel economy
1. Fill the fuel tank completely and record the initial
odometer reading (in kilometers or miles).
2. Each time you fill the tank, record the amount of
fuel added (in liters or gallons).
3. After at least three to five tank fill-ups, fill the
fuel tank and record the current odometer reading.
4. Subtract your initial odometer reading from the
current odometer reading.
5. Follow one of the simple calculations in order to
determine fuel economy:
Multiply liters used by 100, then divide by total
kilometers traveled.
Divide total miles traveled by total gallons
used.
Keep a record for at least one month and record the
type of driving (city or highway). This will provide
an accurate estimate of the vehicle's fuel economy
under current driving conditions. Additionally,
keeping records during summer and winter will show
how temperature impacts fuel economy. In general,
lower temperatures give lower fuel economy.
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