Working As Designed: MPG numbers don't match [non-bug]

[DirtyDawg]

Hadn't even noticed the difference between those two figures, as I rarely see/use the Dashboard feature.

My bigger gripe, which I addressed with Fuelly in an email conversation with "Robert V" on 2/12/14, is the method used to calculate Avg MPG. It appears that the calculation routine averages the MPG figures from each fill-up to arrive at what they call "Avg. MPG." However, this arrives at a fictitious number, or at least a false impression, depending on how one interprets the expression, "Avg. MPG."

As I pointed out in February, averaging the MPG figures calculated from each tank is averaging a list of averages. The more layers of averaging, the more distortion of the actual picture. When averaging the averages, the reported figure will be skewed by how often one fills up -- i.e. on "E" or at 3/4, 1/2, etc. (and more importantly, how consistently one fills up) -- as well as the kind of driving which took place during each fill-up. The error will be more pronounced, the less consistency there is in one's driving demands.

In other words, if each fill-up is not fairly close to the same figure (perhaps due to changing demands, such as long business or vacation trips vs. stop-and-go city driving), then the "Avg. MPG" figure will be skewed in favor of whichever style of driving is the most common/frequent, rather than presenting an accurate picture of the vehicle's real average efficiency. This is because the present calculation is averaging the results from fill-ups not true miles vs. gallons. The better method, as I also pointed out in that conversation on 2/12/14, would be to calculate TOTAL miles driven per TOTAL gallons consumed. That would be a true average MPG over the life of the vehicle.

Just as a specific example, one of my vehicles currently is reported by Fuelly as having an Avg. MPG of 13.4. In fact, when TOTAL miles are divided by TOTAL gallons, the true average MPG over the life of the vehicle is calculated at 14.1. I know, what's 0.7 MPG among friends, right? In this case, it's 5% error. The point is, if this app is to be useful, the figures must be accurate; if they are not accurate, then they are meaningless and useless at best.

I had hoped this would have been addressed in the recent update (as Robert V. suggested it might), but it evidently was not, unfortunately. It would be great to see Fuelly correct this glaring flaw in an otherwise very good app.