There is a bizarre logic in there... it makes sense if
a) the price per gallon is the same, and
b) you could know for certain the 'low fuel' light came on with EXACTLY the same volume of fuel remaining in the tank, and that you filled IMMEDIATELY upon seeing it light up.

Being that that is not realistic at all, what you really need to be doing is

a) fill to shut-off at a specific pump - and note your odometer reading...
b) then drive until you have used about 3/4 of a tank
c) refill to shutoff, at the SAME pump, and note your odometer reading

At this point you can determine the distance travelled, and since you now have a fuel volume to put against this, you can work out your mpg.

And you should be reasonably close to reality if you filled to shut-off at the same pump and filled at least 3/4 of a tank...

Thats what i'm doing now but going out of town this weekend and will need gas on the trip home. i've only used 1/4 tank, so it's kinda early to do a refill. whixch mean when i do refill won't be at some station/pump. i could refill right before my trip but wouldn't take more than 3-4 gallons. might just do that for a short accuracy check and when back from trip (300 miles) check again.

That would be a good idea. You also should begin tracking your FE over a loger period of time as well, so you know what you are averaging overall.

If you do your fills as you suggest you might, you should also be able to get a quick reference against the Scanguage. Just ignore the fillup 'adjustment factor'... simply hit 'done' when you do a fillup so it starts tracking a new tank (or partial tank).

Well I record the reading of the pump and the ScanGauge calculated use and know that the tank only holds so many gallons and it has in the past taken 10 gallons to fill when on the low light so if it consistantly takes less gallons to fill than expected then it should be pretty clear something is going on. Cold weather for the weekend and I will be gassing up in Newport Maine HA HA so I should get a cold tank full yielding even better gas mileage. Other than that it would take a 1 gallon container or a bigger calibrated container. Anyone want to purchase 1 gallon of slow pumped gas and then measure it later in a calibrated container?

Ya know on the low fuel light it actually may be pretty accurate because filling can take half a gallon or more or less depending upon how long you keep pumping in that last squirt to top it off whereas running down to reserve is probably going to be pretty consistant and not off by half a gallon. Of course it does depend upon how accurate they measure the botton of the tank. I guess you could put less gas in when it lights up and test it several times by only adding a gallon or two each time and keep running it down to empty.

I am wondering about the SG not measuring the fuel used accurately maybe something is going on with the engine braking when it measures fuel being injected but it really is not injecting fuel and that causes it to think you have used more than you actually have used. May have to do a bunch of down shifting on a tank of gas and see if it really throws off the measurement.

The only way to get it close IMO is to do full top offs at the same pump all the time. In my case it still shows to be off here nad there by a 1/10th or so.

Another thing Ive found is tire wear starts playing a part.

Ive never had mine be 100% spot on. Theres always a small diff. between the SG tank readings and the hand calc's for fuel ued or the total miles can be off a bit and fuel right,,,, Yada yada yada.

For me its not worth the fiddling with to try and keep it on the bubble. But yet accept it for what it is and use it as a vauble tool to fine tune, adjust or test diff. in me and my tatics.

Its a great tool! But it aint no lab insterment. :)

I'm gonan fiddle with mine a lil and see what it shows for a few fillups, but i agree it'll probly never be spot on and will always reguire a diff corrrection

Where I'm at with it now is:

a) Calibrated with GPS to match as close as possible the real distance travelled. I ended up adding 5% to make it match.
b) I am NOT adjusting any fills within Scanguage. I am however using the same pump, same direction, filled exactly to the same level (in so much as that is possible - I have a diesel - so that makes it easier)
c) I will compare tank by tank fills against Scanguage, and keep an overall number going... this will confirm the accuracy...

It seems to me that if you DO NOT adjust the speed to a GPS, then you end up adjusting the fillup, and accomplish the same thing (the FE number). I guess I'm just trying to make a perceived ambiguity in the user guide something concrete (without trying to create a lab instrument).

Nope the fillup adjusts the MPG by tweeking the amount of fuel you are burning per injector pulse. The speed calibration with a GPS will correct for the speedo error derived from the car ECU speed sensor. Both will affect the indicated and average MPG that it calculates.