Originally Posted by johnnyszx5
It does not use odometer readings, trip or other to calculate mpg. It uses data from the ecm from injector pulse timing and probably some other data to calculate avg mpg. It is by no means precise but it is designed to be fairly accurate.
Yes it uses data from the ECU and one of those signals is the one that drives the odometer display so it really is using the odometer. When you reset a trip counter the current odometer reading in the ECU is stored in another memory location. The unit then looks at the current odometer reading in memory and subtracts the stored trip reading for the trip distance and for the numerator of the mileage calculation. For the gallons, the total sum of the duty cycle of the injectors is used to calculate the gallons which then becomes the denominator in the MPG calculation.
FWIW, I have had (4) Chrysler products that have had trip navigators and the average fuel consumption read out has been within .1 or .2 mpg of the calculated full up to fill up mileage. so the ability to make these things quite accurate exists, Ford just hasn't done a good job with theirs.