I agree, with the possibility of variations between filling nozzles skewing your calculations, your ECU's calculation is almost certainly more accurate. It is, after all, based on an algorithm which factors in the amount of fuel it is consuming/injecting...

I always managed great interstate fuel economy. But it always got ate up between the stop lights. Lol!

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on my flex, I reset the calculator at every fill up (I only fill up, no partial tanks). the car computer is always within .5 mpg of me hand calculating it.

On my 2013 SHO PP I do 25mpg at 75mph all day. Thats using 87 (Low grade). My combined is closer to 19-20. I drive like the old people in Florida, slow and in the left lane.
I will be curious what I do once I get all my mods done in the next 2 weeks. I'll repost once I get some good data.

I've also found that my on board computer is always within half a gallon or less.

Quote from: vernonator on July 16, 2015, 11:01:42 AM

Quote from: ZSHO on July 16, 2015, 09:06:39 AM
I certainly wouldnt rely on the cars computer to give an accurate estimate,calculation based on your mpg,find it kind of obscure in that dept,similiar apps like torque pro seem to be more beneficial for a variety of purposes. Z

Ive done the manual method as well - besides being a PITA it can be just as flaky. Did you use the EXACT same pump at every fill? If not did you fill until the pump auto-stopped? Are they all set at the exact same backpressure? So are you getting the same amount of gas every time? Almost certainly not.... Its all a best-guess and by using the carputer its consistent.

For those filling stop variations, the pumps I generally use after it stops the first time, I go 3 clicks. On the 4th click gas runs over. At every station I have been to. So I am pretty confident my level is consistent. I do leave the cars long term calculator running. But as I recall it shows roughly 17-19 mpg. And based on cars mileage from time I bought it, my records are more gooderer. Besides, I can extract the info for my own uses. But I think the dealer uses this info if anyone ever comes in complaining about vehicle mileage.

The total time to input data into a-car is roughly 15 seconds.  3 boxes that contain current mileage, cost per gallon and total cost. It keeps a running total from there.

Maybe since I only drive mine at best once a week, I am hurting my overall mileage. I maybe once per quarter take a 500 mile road trip so that could be where my losses lye.