Because you are using odometer rather than trip tanks, it's the 33.85 litres from your very first fuelling. It is not included in the calculation because it didn't know the odo reading at the start of the tank (i.e., it knows miles from the end of that first tank, to the end of the most recent tank, so it only look at the fuel used between those points).
The 42.4mpg calculation is correct, but to make the figures make more sense, you could edit the first fuelling so that it says zero litres.
Alternatively, if you know the miles driven for the first tank, you could insert a new fuelling prior to the first (again, with zero fuel).
Either would mean that the total fuel and the total miles recorded in fuelly are limited to the significant readings only (currently, the 'total fuel' is misleading because it includes the fuel from the first tank which cannot take part in the mpg calculation).
MMUK, if that's the case, shouldn't it be part of Fuelly's calculation for MPG to ignore the volume of the first fill up for vehicles that use odometer? I use odometer tracking on both of mine because my scooter doesn't have a tripometer, and when my wife gets gas she often forgets to reset the tripometer in her car.
You say the 42.4 mpg is correct, but it isn't. It's using X number of fillups for the volume, but X-1 fillups for the distance. Sounds like a calculation error on Fuelly's end to me.
Thanks for pointing this out lanh, I'm going to edit my first fill-ups to be 0 gallons.
>... shouldn't it be part of Fuelly's calculation for MPG to ignore the volume of the first fill up for vehicles that use odometer? ...
Exactly, that's precisely what it's doing.
> ... It's using X number of fillups for the volume, but X-1
>fillups for the distance. ...
Nope. When it is calculating the MPG, it is using X-1 for both. Hence you get the correct 42.4mpg.
However, when it is showing you total distance and total fuel volume... it is showing you X-1 for total distance, but X for total fuel volume. That's why when are you trying to calculate it manually, you are getting the wrong number.
Ok, I see what you're saying MMUK. If you manually calculate it using the total gallons and total distance fuelly displays, you wont get the same number fuelly shows as your average MPG because fuelly is completely ignoring the first fill-up, but it's still taking that first fuel-up into account for things like total gallons, price, and number of fill-ups.
It took me a while to figure out the problem Ian had run into, because I use the trip-meter method instead of the odo method. When using the trip meter method, the fuelly totals for fuel & miles are consistent with the mpg. (I don't think it's possible to swap between the two methods).