I have noticed that there seems to be a 'bug' in the 3.61 software. AVG MPG continues to be calculated with the engine shut off and ACC power on. To test to see if your Magnum has this symptom 1. reset the AVG MPG 2. Drive a short distance and note the AVG MPG. 3. Park, shut the engine off, listen to the radio a few minutes and observe the AVG MPG It will steadily decrease just like you were sitting at idle.

 

Also have 3.61 and I noticed that happen too a couple of times. But it happened to me within 10 seconds of turning the engine off. (I've never paid attention beyond that)

I don't think anyone knows for sure how avg MPG is calculated. My guess is that it subtracts your current fuel quantity from the amount of gas you had when you last reset the system, and takes miles travelled divided by that number.

If you go into diagnostic mode, the left and right fuel quantity numbers will change when you go around turns, etc. I've also sometimes noticed small changes in fuel quantity after the engine was turned off. My theory was that as the fuel stopped sloshing around after the car was parked, the system was able to get a more accurate total quantity, and that ultimately affected AVG MPG.

Then again, I could be way off base and it's just a 3.61 bug. I thought I had seen someone else post something similar a while back.

EDIT: Oh yeah, nice pics from your sig link!

 

The computer knows how much fuel the injectors are pumping into the engine and compares that to the miles you drove with that amount of fuel. To rely on the fuel level to adjust for consumption rate calculation, then the mileage would take longer to update and would be very unreliable.

 

Thanks Fred, I had wondered if it relied more on injector pulse, etc. more than fuel quantity.

I like your answer better.

Besides, fuel quantity would have to be measured down to the, I don't know, tenth of a pint or something to give much of an accurate reading for AVG MPG.