I just installed a economy/mileage/automotive diagnostic app called "Garmin Mechanic" It works using the GPS/ accelerometer only or with the automobile's computer via a bluetooth adapter.
I don't have the adapter so I am using the GPS/accelerometer. I hear it is not as accurate using this method. But, the adapter is $90 USD.
This is one cool product ... if it works accurately.
Anyone using this app with the bluetooth adapter? Comments?