Ford F150 1997 Owner's Manual page 192

Maintenance and care
2. Each time you fill the tank,
record the amount of fuel added
(in liters or gallons).
3. After at least three to five fuel
tank fill-ups, fill the fuel tank and
record the current mileage
reading.
4. Use one of the following
equations to calculate fuel
economy.
Liters used x 100 Total
kilometers traveled = km/liter.
Total miles traveled Total
gallons used = mpg.
5. Keep a record for at least one
month. This will provide an
accurate estimate of the vehicle's
fuel economy.
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