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Originally Posted by peid
True, but it will get you within .25-.50MPG, isn't that close enough? It may be the difference between 23.276 and 23.500 or something like that.
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How did you get the .25 to .50 mpg number?
Let's you typically let the gas fill to the point where the auto-shutoff kicks in. I'm sure that varies from station to station and pump to pump at exactly what fuel level that happens, but just for example....
Now you let your wife fill up, and she tries to squeeze in as much fuel as she can on top of that. That can be several extra gallons, or 10% of the tank.
If you plot out your gas milage fill by fill, your wife's fill will have great gas milage, and the next one will be lousy. That has nothing to do with the real gas milage, it's just that you've introduced error in the actual # of gallons consumed during that mile interval.
Or, let's say you always try to pack in as much as possible. One day you fill fuel that just got delivered to the station and is 70F. The other you go to a station and fill from a tank that's been in the ground a while and is 50F. When that 70F fuel cools, you are left with some void in the tank because fuel contracts as it cools. If you're not a firm believer in the expansion of fuel with higher temp, consider the bleed tubes on airplane wings so that fuel can leak out when they got hot. That's why we don't fill light airplanes to the top. Most car gas tanks have a void somewhere to accept expanding fuel instead of dumping it on the ground (good idea I think!)
So, things that give you different amount of fuel on a full tank:
-temperature of the fuel
-sensitivity of the auto-shutoff
-slope of the car (or)
-how much of the tank's air pocket gets filled
If you could be certain you always fill to "the top" at the same temperature, you'd get good milage calculations, but it's not possible as far as I know.
The only way to get accurate numbers tank by tank is by using averages, or by using high quality fuel flow meters before and after the injectors. Good flow meters are about $2k/piece so I'm avoiding that...