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Old 10-22-2015, 06:52 AM   #1
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Question about mpg calculation

Hi everyone,

I'm new to Fuelly and i have a couple of stupid questions for you:
  • Why do I need a full fillup in order for mpg to appear?
How is the mpg calculated even after a full fillup? I'm asking this, because in my case I have travelled 491km with 39,6L (partial fillups) till my first full fillup. If you calculate the mpg is about 8,08 L/100km. However, the Fuelly calculates the mpg till then around 8,72 L/100km! I have attached a screenshot as well.

Probably this is something I'm missing in the way Fuelly calculates mpg.

Thank you in advance for your time.

John
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Old 10-22-2015, 07:11 AM   #2
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A fuel is the fuel you just used, over the km's you just drove, it's impossible for fuelly to know how much fuel you used if you only put a bit in at a time. Going from full to empty to full again is the only accurate way to measure your fuel consumption. I think you may have forgot to check the partial fuel up option in that fuel up you are questioning?
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Old 10-22-2015, 08:33 AM   #3
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Thanks for your reply Draigflag.
I understand now why it cannot calculate the partial fillups. I thought that it just calculated the mpg per Liters of each fillup. I was wrong.

However, the fillup in question was not a partial but a full one. It's just hard to get how the mpg is calculated from a full fillup to the other (it's mentioned that the preceeding partial fillups are included). I would be grateful if you could explain this.

John
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Old 10-22-2015, 09:24 AM   #4
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My maths isn't strong enough to help you on that one, but I'm sure someone who knows the answer will be a long soon. Sorry I couldn't help more.
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Old 10-22-2015, 09:57 AM   #5
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There is a lot of variance between a "full" tank and a "FULL" tank. Some tanks can hold another gallon or more even after the pump has clicked off, depending on their shape and size. Ambient temperature and ground temperatures around the pump reservoir affects how much space the fuel occupies. It's nearly impossible to fill your tank to the exact same spot every time, since you cannot see the level, and the fuel density differs from moment to moment, from pump to pump, from day to day or season to season. There will always be a little inaccuracy associated with fuel economy calculations on the road, unless you're in a climate-controlled and minutely-metered science lab.

We do the best we can, and we live with it. I think you're expecting too much.
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