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Re: The guage works like this - (archive)

[ Follow Ups ] [ 5-series (E34) Message Board ] [ Msg. Board FAQ ]

Posted by Andre on November 04, 1999 at 07:47:53:

In Reply to: 5-speed owners, does your car do this? (Wierd...) posted by Tim Collins on November 04, 1999 at 00:40:10:

The guage is a relative (ie, not exact, but fairly close) indicator of the MPG. Based on the electronics involved, it appears to be taking a signal from the DME relating to fuel flow and using the MPH, figuring a short term avarage that it displays. Visual observation and a knowledge of electronic leads me to believe that it is using about a 4 sec. "averaging" or response time.

As for why it reads higher in gear than in nuetral when decelerating, the DME cuts off the injectors when the throttle is closed above a certain RPM (~1300 for my DME / emissions package), and below that turns the injectors back on when below that to prevent stalling. Watch your tach, the MPG guage, and listen to the engine and you will see/hear this. So, when decelling in gear at 2500, your car is using NO gas - good for emissions, your wallet, and engine braking - clever, huh?

: Ok, maybe I just have NO IDEA what the hell I'm talking about when it comes to car engines, but I've noticed a strange behaviour. Say I'm driving @ 40 MPH. I take it out of gear, and leave it in neutral, so it's just cruising and idling. I would think that the mpg needle would show the top, because I'm using just enough gas to keep a 700 RPM idle and going like 40 MPH, no? It doesn't. It is high, but not at the top. But if I have it in gear and just let completely off the gas, it DOES go to the top (highest MPG, no actual #). Why? Isn't it using more gas to maintain, say a 2500 RPM idle (if I leave it in 3rd) than a 700 RPM in neutral? So shouldn't neutral, 700 RPMs give me better mileage? I just find this really strange... can someone please explain this to me?!

: Thanks!

: Tim Collins
: '90 535i 5-speed sharked

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