slinky22 - i do the same thing with one of my cars, but because there is a slight fuel leak which is exacerbated by a full tank. Every time I get close to empty, I put 5 gallons in the car, reset the trip-meter and calculate the fuel economy for that period based on 5 gallons consumed. While there may be significant error for each individual fill-up, as you continue to drive your car and add more fill-ups, your overall average will be accurate. If you select the "partial fillup" box, the site will not track your data, but if you don't select that box, it will track the data and after enough fillups, your average gas mileage will be correct.
Also, to the admins - I noticed the new averaging algorithm has taken effect both when I go to the vehicle itself and when I go to my profile, but not if I go to my dashboard (http://www.fuelly.com/dashboard/).
slinky22: From what i've heard, that will actually lower your gas mileage as it increases the evaporation rates, which makes sense because more gas would be needed to vaporize in order to equalize the pressure.
stevarino - Assuming his van is fairly modern and in fairly good condition, fuel vapors from the tank will be stored in the carbon canister and then fed to the intake manifold during normal engine operation. As of the 1970s, vehicles aren't allowed to vent gasoline vapors to the atmosphere (at the time that law went into effect, this type of gasoline vapor counted for about 20% of harmful vehicle emissions...ouch!). These vapors will then be combusted, and the O2 sensor / EFI will adjust the fuel injected accordingly. However, by fueling up to about 2/3 of a tank (assuming a 20 gallon tank), he is only saving about 40 lbs of mass, which in a 4000+ lb vehicle probably doesn't amount to a whole lot of fuel savings.
God you guys are nerds....no offense, but its just a website that im pretty sure wasn't designed to hold up to the scutiny of the scientific process. have a little fun guys....and stop being such killjoys....
My mind started ticking after reading this, as I have the same problem. I'm paid monthly, and I do a full fill after payday, another full fill once it gets low, and then I limp along with partial fills till payday.
What I was thinking is that if we can track partial fills (as we can now), then counting the total milage and fuel volume between full fills, we can include partial fills in the overall calculation once there has been a full fill.
This spreadsheet (stolen and modified from above) shows what I mean.