10-tank average calculations are wrong on Fuelly

[SteveMak]

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.