I would imagine having your A/C, CD player and butt-warmers on, and their impact on mpg (if any), would be accounted for by the computer anyway.

Yes we've seen descrepancies that big between the computer and the "by hand" method before. Sometimes it ends up that "by hand" is the larger number of the two!

Like Schwa said, there's a margin of error in both methods. The computer has to have some degree of error, and whatever gas pump you go to may not shut itself off at the exact same time every time you fill up. Or if you go to different pumps routinely, or if you add a lot more fuel after the first click.

There are a lot of variables there...... with our HiHy, we just try to be consistent so we don't add variables on top of variables. We stop re-fueling on the first click, and occasionally our "by hand" calculation is actually higher than the computer says, but most often it's at least 1mpg less. I'll think we're going to have a tank that's 32mpg or higher, only to find out it was 30.5, etc. Or, if "by hand" is just as inaccurate as the computer, then I can assume I really *did* get 32mpg or higher!

Rick