10-tank average calculations are wrong on Fuelly
 

I'm a stickler for accuracy, so I have been using both Fuelly.com and my own spreadsheet to track fuel economy. I have noticed an error in the way that Fuelly calculates the 10-tank average fuel economy.

According to my observations, it appears that Fuelly simply takes the L/100 km value for each of the last 10 tanks, and averages those 10 numbers. I assume (though I have not proven) that it does the same for MPG. In my specific case, when I average my last 10 tanks' L/100 km numbers using my spreadsheet, I get a value of 6.67 L/100 km (rounded to 2 decimal places), and Fuelly displays 6.7 (rounded to 1 decimal place), which supports my conclusions.

What Fuelly should be doing, is:

1. Add the total distance traveled over the last 10 tanks
2. Add the total fuel consumed over the last 10 tanks
3. Used the values in #1 and #2 to calculate a true 10-tank average L/100 km or MPG.

When I do this in my spreadsheet, I get 6.57 L/100 km (rounded to 2 decimal places), which Fuelly should display as 6.6 (rounded to 1 decimal place). In my case, the difference results in a 1.5% inaccuracy, but it could get worse if you drive significantly difference distances when you fuel up and/or if you get significantly different fuel consumption per tank.

I hope that the folks at Fuelly will look into this, and make the fix.

Thanks for the post!

It's a topic that has been discussed before both on here suggested by users and internally among the Fuelly team.

Currently the Dashboard last 10 is calculating like it has always been (from when Fuelly was originally built).

We do plan to revisit it in the future after things slow down with our mobile app integration.

Cheers!

I've been informed this has been updated. Please take a look now.

Separate issue perhaps, but I noticed on "recent activity" on my dashboard, my second most recent fuel up says I got 53 mpg, when I actually got 57 mpg. According to the rest of my data it's correct, it's only in the "recent fuel ups" list at the bottom of the page it displays incorrect. I'm just wondering if I actually made a mistake and then corrected it but it still shows up in recent fuel ups.

I think that's the case. It's capturing your original entry and not being updated if edited.

Ah ok cheers, just checking it wasn't a bug and wasn't contributing to the rest of my data :)