So I filled my tank and calculated the mileage - a horrible 12 litres / 100km or about 20 mpg. This is much worse than my last few tanks without any significant change in my driving patterns. One thing I noticed during that last fill is the pump was very slow. So that got me thinking about differences in pumps, the long fuel neck of the Fit, and how this affects mileage.

I always stop at the first click. But on this last fill, it didn't click but I could hear the gas sloshing in the neck so I stopped. I figure I filled up much fuller than in previous fill-ups. As you may know, the auto-shutoff on pumps work on a vacuum principle - if you don't know what i'm talking about, then read here:

Life After Coffee How does the gas pump shut off

So I think the shutoff typically kicks in when the gas sloshes up to the vacuum hole. If the pump is fast and causes lots of sloshing, it'll click early. In my case, a very slow pump means little sloshing so I was able to fill the neck right up. So I may have overfilled 3-4 litres, which would explain the crappy mileage on the last tank. The good news is that I should make it up on my next fillup if I don't overfill again.

Moral of the story?
Different pumps will shut off differently - faster pumps will likely shut off faster. For those of you who like to fill it right up, ease up on the pump to keep the stutoff from kicking in. And no matter how you like to pump, make sure you consistently pump the same way every time or your gas mileage chart will look like a yo-yo.

So does any of this make sense or am I out to lunch on this?

 

It makes a lot of sense!
That is why MPG for any one tank full is not all that accurate.
It needs to be averaged over several tanks IMHO

 

Thanks tork. I guess everyone else on fitfreak thinks I'm out to lunch

I usually do several tanks before calculating mileage, but I've been calculating after each fill-up for the Fit since I got it so I could see how good/bad the mileage was and how good/bad the fuel monitor was.

 

i always use the slowest pump setting cept at oone place or else it won't auto shutoff. also I try n use the same pump each time but i figure as long as i still break 30mpg i'm happy during the winter but through the summer i usually pay more attention to my mpg.