There is a lot of discussion regarding the on board mpg display, and how accurate or otherwise it is. I wanted to tell you about something I noticed, yesterday. When I started out my on board display was showing 51.1 mpg since last top up. My ScanGauge E indicates the average mpg for the current journey (as well as storing other trips, including the current tank). The ScanGauge E tends to be a lot closer to the calculated values, even though it gets its data from the same source as the on board display. This better accuracy is down to the fact that a correction factor is added (if required) at every top up.
What I noticed on yesterday's journey was that the on board had started to climb above 51.1 even though the ScanGauge was still showing less than that figure. In an ideal world, with accurate information, the on board would not rise until the ScanGauge was better than 51.1. In fact, it should have possibly dropped.
Even my calculated values show variation. I do my sums, as I have done since they started selling petrol in litres. I take the litres, covert them to gallons (to one decimal place) and divide that into the odometer miles minus previous odometer miles. It is how I have always done it. Fuelly, which I also use, does the calculation using litres without the conversion to gallons. That is more accurate and shows marginally better mpg than my calculation!
Even the miles can be inaccurate. If you use the trip miles they can be 425.4 miles but the odometer mileage would round that to 425. So mpg is all in the eye of the beholder and how it is calculated. Only thing it shows is whether it is better or worse than last time!!