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2003 BMW 5-Series 540iA 4dr Sedan 5-Spd Auto

When I calculate the fuel economy, I always get a lower number than shown by the car odometer. When I fill up the tank, I reset the trip meter to zero, and when I fill up again, I divide the trip mileage by the number of gallons needed to refill the tank to arrive at the mpg. Why is the odometer mpg always better than my manual calculation? I have 14k miles on the clock.

Mechanic Q: How is the mileage (mpg) calculated??

When I calculate the fuel economy, I always get a lower number than shown by the car odometer. When I fill up the tank, I reset the trip meter to zero, and when I fill up again, I divide the trip mileage by the number of gallons needed to refill the tank to arrive at the mpg. Why is the odometer mpg always better than my manual calculation?