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Did you try to reset it via the steering wheel. Hold reset for about 5 seconds. If that does not work pull the negative on the battery for 10 seconds and then re-attach. The mpg is calculated by fuel consumption I am sure.
Average is calculated over all mileage. Current is measured after each fill-up of gas. Press the "Reset" button (on the inner right side of steering wheel) to clear the stored data. Or, disconnect the battery for 5 minutes for a soft reset of the settings. (Disconnecting battery loses radio station presets and some saved Navigation points.)
If you disconnect for 10 seconds to a min or so it does reset but you don't loose radio stations. At least in my car. As far as an nav presets go I'm not sure as I don't use them but I never loose radio stations.