Originally Posted by F250_
Well, Mark, I will say that when you see gas ready to flow out the top of hte fill neck, and do that at every fillup, you can fill your tank more accurately than the OEM meter can calculate. My basis is the fact that there are some assumptions built into the meter's calculation regime that can change based on driver, geography, driver's mood, etc. That's why I always use only hand calcs myself, because I fill up until I can see the fuel... that way I know that I'm getting the same amount of fill, regardless of which station or pump I use.
There are obviously some folks who get very good and consistent results with the meter, but my own experience hasn't proven it to be reliable for me, and most who compare it to hand calcs find the same to be true for them as well. Again, though, I don't doubt at all that it does work for some, like yourself and a few others here who say they have verified it for themselves.
I was just reoprting my experiences and opinion. But since you brought it up, you said the OEM meter is making some assumptions and you're right - but when you calculate by hand you're also making some assumptions, like that the pump displayed the correct amount, your speedo is accurate, your truck was nice and level at the time of the fill up, and as ohers mentioned, the gas was cool and bubble free.
When you think about it - @ 44 gals of fuel, if the pump is inaccurate by +/-1%, then your off by almost 1/2 gal (+/- 0.44 to be exact). Now let say that you're getting 12 mpg and you refill after 480 miles, when you calulate you could be off by as much as +/- 0.15 mpg (you should have gotten 40 gallons, but that 1% could put at 39.5 or 40.5, depending on the + or -). Now if you throw in your speedo being +/- 1%, you've just increase the variance and your mileage could be +/- 4.8 from the actual odo.
There are just so many variables that come in to play for both methods, hence the reason the goverment chose the term "Estimated MPG".
So my point was that I felt mine was close enough for me to depend on - but I still recheck it from time to time when were on trips.