In order to calculate MPG, we use the values from the Mass Air Meter and then calculate back from there to get mass fuel used. It's really a simple calculation, but can be affected by things such as Ethanol which not only has a slightly different mass per gallon, but also burns at a different stoichiometric ratio. This can cause a slight shift in the accuracy of the Avg. MPG readings versus hand calculated.
As for the speedo, yes... keep speed dead on with the GPS. All the other calculations (¼ Mile, 0 to 60, and Economy) require this to be as accurate as possible. This also helps keep the odometer accurate.
Enjoy all.
__________________
Bill Cohron - The Mad Doctor  
Power Hungry Performance - The ORIGINAL in Ford performance tuning... Since 1997!
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