Last night I noticed that if my key is turned on so that you can read the gauges, but the engine is not started, my MPG actually started to decrease! It made a hugh jump from 34 to 30 fairly quickly, and actually (over much time), got down to 27 without the motor running! Is this normal?

 

The computer gets MPG by dividing average MPH by average GPH. MPH is your average speed.. GPH is your average fuel consumption in gallons per hour figured by injector duty cycle.

 

So...even though the engine was NOT running, and no fuel was being consumed, the computers clock was running?

 

it's not accurate... it continues to recalculate even when vehicle is idle... I think it just computes your miles vs the time you've been running and estimate the consumption from that... it might factor in the amount of fuel remaining in the tank, but I doubt it's that sophisticated

 

it looks like the computer does use a sensor in the tank, as im filling up i can watch the Distance to empty increasing... ( engine off)

it also changes depending on the slope im at.