Maybe this has been addressed before but I could not find any threads. I am wondering why the mpg displayed on my car's (Honda Insight) MID when I fill up is slightly different +/- by a couple of mpg's than what Fuelly says it is? So, for example on my very first fill up, my car showed 41.4 mpg but Fuelly calculated it as 44.6. On my fifth fill up, my car showed 41.7 mpg but Fuelly was 48.9.
I know the MPG thing on my older caravan never matched. But then I reset it at every oil change whereas Fuelly doesn't reset as it is a true average. And it is simply going by the distance you drive and the amount of fuel you use. The individual totals are averaged. No one fills their tank the same exact way every time. I usually round mine up to the next buck or two, if it lets me. That can cause discrepancies on individual fillups, but not your overall average. The meter in the car usually goes by what it thinks it should be. It cannot physically measure the amount of your fuel by unit. It measures mostly by usage and is only an estimate, not exact.
I will agree with mechjames, check your site location settings, make sure you have it on the right country.
It's difficult to get an exact reading every time. Are you filling up to the top every time? If not, your fuelly.com calculation may be off. Since it uses the number of gallons you put in your car, you need to know that you're being consistent on your fillups. As for what your car tells you, I've had a number of cars that told me my estimated MPG and it's never correct. If you're getting more than what it says, then it's a good thing!
Looks like some of the computer's guesses are high and some are low, according to your gas log. That is probably just because it's very hard to fill the tank to the same volume at every fill up. Even a couple tenths of a gallon difference can mean several MPG difference in "measured" fuel economy, especially on a car that uses very little gas. If you've made sure you're using USA gallons instead of UK gallons, I'd try averaging the Honda computer's guesses against your measured fuel economy over 5-7 tanks. That will tell you how accurate it is.
For the last 8646 miles i've tracked on fuelly, the computer reading on the car has always been a consistent 2 to 3 MPG out every fuel up!
Fuelly is more accurate of course, but only if you brim the tank in the same way every time, for example, i always fill right to the top so there is fuel sitting in the nozzle end, that way i know it's as full as it's going to get!
Others pointed out why Fuelly is more accurate than any car's trip computer (so is paper, pen, and simple division), but if you really want to get accurate, try to use the same pump at the same gas station as much as you can.
Even pumps at the same station can vary between each other (metering error), and each nozzle has a different "click off" threshold, which can vary -quite- a bit.
The mpg computer in my car, and every car like it, is 2-3mpg optimistic.