I put 8.1 gals of gas in my '04 this morning. The MFD average MPG reading at the time was 51.1MPG. However, when I divided the gals added by the total miles driven, my average read 46MPG. Last fill-up, I had just the opposite experience. The MFD reading read 48MPG average but the my gals added divided by my miles driven calculation said my average MPG was 55MPG!? Which method is the most reliable for getting a realistic MPG assessment?
It depends a lot. Over the life of your driving the miles driven over the gas burned is going to be the most accurate (given some innaccuracy both in the odometer and the fuel pumps). On an individual tank basis the MFD is probably, in most cases, a little more accurate since it is a measure of the fuel used (indirectly). Each individual tank can vary on the miles over fuel used calculation due to shrinkage or expansion of the bladder, cut-off point for the gas pump, over or under filling on your part, inaccuracy in the meter on the fuel pump, etc. Those minor innaccuracies tend to balance over time.
if you use the same gas pump all the time then you will notice a fairly consistant difference over several tanks. as a per tank basis, your fillups will not be close enough for you gauge accurately. you should not be topping off your tank. but the mileage you are getting is so high that even a very little amount of gas can change your stats quite a bit. that is why you will get a see-sawing of which measure is higher. i still take the MFD to be more accurate as its more consistant in its measuring. use the MFD to determine which pump gives you the best mileage and stick with it. that is what i did.
I use the MFD calc to tell me the minimum I should be able to get in as I equate that to how much I used. Even if the tank was only 90% full to start, I should be able to get that much in again. I can't start with the tank over 100% full! ;-) Of course, with only 2 fills under my gas cap, I don't claim to be any sort of expert.
