Total avg mpg calculation?

[BDC]

I exported my fuelups for my Scion and calculating it myself in Excel (total miles divided by total gallons) says the lifetime average is 28.9mpg. Fuelly says its 29.2mpg. What do you make of it, PB?

Could it be a rounding error somewhere that Fuelly includes but Excel does not? For the record, I have no fuelups without both miles and gallons.

[pb]

According to your fuel-up log, your Scion has these numbers:

Total Miles: 40058.4 / Total Gallons: 1371.050

That gives me 29.2 on the calculator. Looks like you started that vehicle with odometer tracking and then switched somewhere along the way. That could definitely throw a wrench in your spreadsheet. Your first odometer-based fuel-up should show zero miles because we have no way to calculate distance on that first fuel-up.

[BDC]

I did originally start with odo logging, but the only spreadsheet I have (the one I exported from Fuelly earlier today) doesn't show that. It looks like it was converted right.

From the spreadsheet, total miles = 40058.4 and total gallons = 1383.86. The gallons in my export file read 12.811 gallons greater than your number. Coincidentally, the only 12.811 gallon fillup I have made is the very first one. How did you calculate the total gallons number? Did you export the fuelup .csv file or do you have an administrator way of viewing it?

[pb]

That's not a coincidence. Fuelly isn't counting your first fuel-up because there should be no distance reported. What was the distance value for the first fuel-up?

[BDC]

295 miles. Why would it not count just the fuel from the first fuel-up? I understand if you buy a new car and the trip odometer is not set, but shouldn't the user understand not to put a fuel-up in with zero miles?

Or is this just something left over because I started the car with odometer tracking. An initial odometer reading would be important to have.

[pb]

So I'm looking at your first fuel-ups here. You created your car on Fuelly and then entered an odometer reading of 405. Based on that, we have no idea how far you drove before you put in those 12 gallons. You didn't drive 405 miles--that was your odometer reading. So Fuelly puts zero as the distance and starts tracking from there. Fuelly should have recorded zero miles for the first fuel-up and zero MPG. There was no way to calculate an MPG for the first fuel-up.

Next time you fuel-up and put in 753. Now we have something to work with. You went 348 miles. Fuelly calculated an MPG.

It looks like what happened is that you had some earlier fuel-ups at Fuelly and it was keeping a running total of distance. Then maybe you removed some earlier fuel-ups, I don't know. In any case, the miles on the first fuel-up on August 1, 2008 should be zero, not 295. Because according to your logs we have no idea how far you went when you put in your reading of 405.

[BDC]

But it says 295 in the export log. I don't recall if I went back and entered that when I switched from odometer tracking to to trip miles or not, but it was over a year ago if I did. Now that I check it in the "View All Fuel Ups" I can't find anything about the 295 miles on the first tank.

I do recall the car had 110 miles on it when it was delivered, which would give us the 295 number. I might have gone back to enter that when I switched, but I don't remember if I did. That's very strange that it will export that number in the csv file, but not on the all fuel-ups screen.

Perhaps I should just doctor up the export file, delete the car, and import all the fuel-ups for a new car, just to get rid of the inconsistencies from the odo-trip miles switch. That seems to be confusing you, me, and Fuelly. :P

[pb]

But it says 295 in the export log.

Yeah, that's where the bug is that I need to track down. It shouldn't say 295 in the export.

We allow people to switch back and forth between odometer and tripometer tracking as a convenience, but it can lead to problems like this. Because the first odometer reading of any series of odometer readings always needs to be discounted because we won't have distance data. As people switch back and forth and then go back and edit/remove fuel-ups, artifacts like this can remain. In the scheme of things it's not a big deal—you're within 1 MPG on your average either way. And now that you know where the problem is you can edit your spreadsheet, and I can fix your data here.

[BDC]

Alright, that all makes sense. I may go back and re-create the car from scratch just to make sure it all works right in the future. If I get bored. Thanks!