Incorrect mpg

[RussTaylor]

Hmmmm, well they've built a decent site I can use for free so I'm not in a strong position to argue, and they know what development they have on the books already so maybe they don't have resources for it, but I think it's a good idea as well

[pb]

Fuelly measures distance using the difference from full. If you do a bunch of partials in a row, how do you know the distance traveled on that fuel? There's no way to do it accurately.

[MMUK]

Paul, Russ is referring to a series of partials followed by a full (and in this scenario fuelly does have enough info to work out the total fuel volume. It is only when the full tank is done that this can be calculated).

[RussTaylor]

Yeah, that's my point, ignore them until you get a full, then just count it as one large tank. This site looks good, I'm sure by using some nice bordering or something you could show that the fill ups are counted as one.

I think this would be a very neat feature, and not something I've seen before. When trying to be economical in my last car I would only ever carry half a tank, saved about 40kg weight all the time dragging that up and down hills and made a difference. I was unable to use an app/website to get my mpg though and had to rely on doing it myself

[pb]

I hear what you're saying MMUK and RussTaylor. But several partials, even followed by a full, will give you incorrect distance information. My point is that you can't count them as one big tank because the size of the tank matters for calculating distance. The difference from full is going to be off if you include partial fuel-ups. The only point we have to measure from is a full tank.

[Ciderbarrel]

What people need to realize is that when you use a partial fill after a full tank, that amount isn't how much gas you used to go the distance on your odometer, it is how much gas you put in your tank. It will be less than the amount you used.

When you do two partials in a row, you will never know how much gas you did actually use and you have zero accuracy in determining how much gas was used to go the distance you are accounting for.

[DoorDing]

"Difference from full" gives a point of reference for calculating fuel economy. With partial fill ups, there's no point of reference to use for measuring the amount of fuel consumed.

The only way around this would be for the user to manually determine how much fuel was consumed without filling the tank. This isn't a matter of adding a feature to Fuelly; any fuel economy calculations require good data, and partial fill ups don't allow for this on standard vehicles. The only somewhat practical way around this would be to install a very accurate, digital fuel gauge in your vehicle. Doing so would allow manual calculation of the amount of fuel consumed. In my opinion, it's better to just fill up the tank if you're really interested in maintaining a fuel economy log.

[DoorDing]

"Yeah, that's my point, ignore them until you get a full, then just count it as one large tank. This site looks good, I'm sure by using some nice bordering or something you could show that the fill ups are counted as one.



I think this would be a very neat feature, and not something I've seen before." - RussTaylor

Sorry, but the reason you haven't seen it before is because it can't work for the reasons Ciderbarrel and I mentioned.

[RussTaylor]

I'm sure you're right, and I'm missing some subtlety, but can you explain where this doesn't work...

I have filled my car up, it took 50ltrs (doesn't matter). On my odo I have 100 miles. So starting point = full tank and 100 miles on odo.

I drive around, and do a partial fill up, so 10 litres, my odo reads 110 miles.

I do the same again, this time I fill up 20 litres, and my odo reads 140 miles.

I then drive around, and perform a full fill up which takes me 30 litres, and on my odo I have 200 miles.

Since the last full tank (our reference, no different than my very first fuel up on Fuelly) I have travelled 100 miles, and used 10 litres + 20 litres + 30 litres.

This is the amount of fuel I have put into the tank to return to our reference point (full tank). So 100 miles and 60 litres...

Sorry to keep bashing the point but I disagree with both of you. You say you have no point of reference, but you do from your last full fill up. You have how much you put in the tank in between, and how much it took to fill it up again.

[Ciderbarrel]

Just because you put it in the tank, it doesn't mean that is how much fuel you burn unless you let to go to empty (the engine dies from lack of fuel in the tank).

You are confusing fuel purchased with fuel used.

Starting point is good 50L and 100 miles.

You drove 110 miles total and used 50L plus some unknown amount, it was more than 10L because you stopped at 10L. You have used 50L + X, the unknown amount

You drove 140 miles total and used 60L + X and now + Y, another unknown amount. You have no idea how much was used because you did a partial fill twice in a row. Did you use 20L? 30L? You will never know for sure.

You drove 200 miles total and used 60L + X + Y and now + Z, another unknown amount.

This is why a partial fill gives you ZERO accuracy for fuel economy.

Picture it this way. You have a 50L Coke bottle (huge, I know). You can not see through the wrapper, nor shake it to test the amount in it, nor weigh it.

You top it off, so it has 50L. You drink some and fill it back to full.

You then drink some amount, not keeping track, in a week and put only 10L back in and it is not full. You then drink SOME more, but you only put 20L back in. It is still not full, and then you drunk more and put 30L back in, it is still not full.

Tell me, how much Coke did you drink? You have NO way of knowing because you have no reference point for your measurements.