Profile averages seem off
 

On the profile page, I believe there may be an error in how some of the averages are calculated.

I have 97,726 total miles tracked and 324 total fuel-ups. This would seem to imply an average of 97,726/324 = 301.6 miles per fuel-up, yet the profile page is quoting an average of 303.5 miles per fuel-up. Since 97,726/322 is roughly 303.5, it looks as though Fuelly may be using a fuel-up count that is short by two in calculating the average fuel-up distance.

Also, the average price per gallon figure differs from what I have in my spreadsheet. My calculation is based on total spent divided by total number of gallons purchased (i.e. $8,052.23/3,067.492 = $2.6250/gal), while Fuelly is reporting an average of $2.64/gal. Perhaps this is because Fuelly simply averages the prices of each fuel-up, rather than keeping a true running tally as I have in my spreadsheet?

Note that the average *total* price per fuel-up *is* correct. I have $8,052.23/324 = $24.85, which is what my profile page is reporting as well.

Anyway, I was just curious about what was happening here. As always, thanks for all your work on the site!

You're using odometer tracking, so keep in mind that Fuelly can't use your first fuel-up for its calculations. It needs a baseline reading to start from and from that moment on it starts calculating the distance traveled with each fuel-up. Using that first fuel-up without a distance would throw the average off. That brings us down to 323 fuel-ups we can use.

You have two fuel-ups on Feb 12, 2010 that have the exact same odometer reading. I'm guessing that was a mistake, and once you remove one of the duplicates that will bring the number of fuel-ups to the right number.

And yes, for average price Fuelly does a simple average of all prices instead of total spent / total gallons. We calculate average MPG using totals, but not prices. We could probably move to that for prices too.

Actually, the two fuel-ups with the same odometer reading are legitimate distinct fuel-ups. I actually had run out of gas, so I had to put in just enough to get going again and limp under a mile to the nearest gas station to fill up the rest of the way. Turns out that the odometer reading was the same at the point where I had run out of gas and at the gas station. Is there a way around *that* kind of situation?

Also, with respect to overall average MPG, Fuelly shows 32.1 and I have 31.97. I'm guessing that this is due to the difference in averaging methods, or due to one or two fuel-ups being thrown out.

Yeah, the average miles/fuel-up is going to be off from your method of calculating when you have two fuel-ups with the exact same odometer reading. Fuelly can't calculate a distance if there's no difference. And if there's no distance, there's no way to include it in the distance-related statistics.

Why not calculate average miles per fuel-up as total miles tracked divided by n-1, where n is the number of reported fuel-ups? This would still throw out the first fuel-up, while simply incorporating "no mileage" fuel-ups as 0 miles (which is really what they are). Obviously, this will make the average miles per fuel-up lower, but the same sort of thing would happen even to someone who fueled up every 20 or 50 miles, right?

People sometimes switch tracking methods between tripometer and odometer, so there's no guarantee that n-1 would always be accurate. Zero miles/fuel-up feels like an edge case to me. We want people to enter fuel-ups with a distance and anything else is going to confusing and hard to explain.