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Discussion Starter · #1 ·
OK...when I approach a stop light or stop sign, I usually shift into second, slow down, and then shift to neutral and use the brakes. But as I'm slowing down, and when I come to a stop, the MPG needle creeps up and up and then finally falls back to 50. Does this happen to anyone else? Why does it do this?

Here's a video:
http://kandeecup.tripod.com/MPG.wmv
 

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I would guess like the engine is like revving or something... something to do with the software..:hmm:
actually now that i thought about it... my car didn't do that at first...
i think after 10k miles it started do it...:hmm:
 

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My friend and I have

hypothesized that the mpg gauge is based on manifold vaccuum pressure, since it moves in much the same way as a vaccuum gauge or a boost gauge with a vaccuum section. Those gauges always flip around when taking the engine through these types of motions. It would make sense in terms of reading estimated mpg too, since effectively a vaccuum gauge can tell you how wide open your throttle butterflies are. Just a thought.
 

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Because instant MPG is calculated using fuel consumption rate and vehicle speed. When you are in neutal the engine is idling so it still consumes fuel. As you slow down fuel consumption is constant but vehicle speed becomes reduced, so MPG approaches zero. Finally, when vehicle speed becomes slow enough, the gauge returns to it's "off" point which is to the far left. It would be a lot more logical if the MPG gauge went to zero on the right, but since the gauge returns to the far left when MPG is zero, it moves right at first then left when you're almost stopped.
 

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Discussion Starter · #15 ·
abomb said:
Because instant MPG is calculated using fuel consumption rate and vehicle speed. When you are in neutal the engine is idling so it still consumes fuel. As you slow down fuel consumption is constant but vehicle speed becomes reduced, so MPG approaches zero. Finally, when vehicle speed becomes slow enough, the gauge returns to it's "off" point which is to the far left. It would be a lot more logical if the MPG gauge went to zero on the right, but since the gauge returns to the far left when MPG is zero, it moves right at first then left when you're almost stopped.
THanks
 

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it always happens to me and is completely normal...i contacted BMWNA and they told me all it is doing is recalibrating its self :thumbup: actually if it isn't doing it you should have it checked
 
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