Kingnutin,If you plan on using the DIC for it, you will have to reset your average mileage. However, it's not very accurate, so you should calculate it yourself. Pick one of the trip odometers to use, reset it when you fill up, then next time you fill up, divide the mileage by the number of gallons it took to fill, that's your mileage for that tank of gas.
kingnutin,Cycle through to the average MPG, hold down the check button till it resets, then you can begin. As it is, there is no way of knowing the last time is was reset, so its still averaging from whenever that was.