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E85 / E86 Z4 (2003-2008)
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  #1  
Old 04-27-2012, 02:46 PM
keenmachine keenmachine is offline
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Mein Auto: '04, Z4
Z4 mpg computer

Anyone noticed the millage given on the dash computer is quite a bit off? It always says 9.3 l/100km = 25.3 mpg. But I actually use 10 l/100km = 23.5 mpg.
What do you guys get and is your computer always that far out (almost 10%)? It really should be much more accurate.
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  #2  
Old 04-27-2012, 07:51 PM
Stromtech Stromtech is offline
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Are you calculating to Imp Gallons or US Gallons? Imperial Gallon is 1.2 US gallons.
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Old 04-27-2012, 08:18 PM
WestCDA WestCDA is offline
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If it 'always' says 9.3l/100KM it sounds like it hasn't been reset for a long time - the mileage is averaged over the period of time the car has been running since the last reset. i.e. if you drive a long time in the city and then take a short trip on the highway, you'll likely see no change in that reading. Reset it and accelerate hard - you should have no problem seeing it hit at least 15L/100KM and up - for a short while.

Reset the fuel computer before a period you'd like to measure. With a light foot and keeping the speed close to the limit, I can get under 7l/100K on a mostly highway trip. That's better than 40 miles per imperial gallon, and even with the top down. On a mix of city and highway, I seem to end up around 8.3 or 8.4 over an extended period of time.
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Old 04-28-2012, 03:39 AM
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Pinecone Pinecone is offline
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Also, they don't actually measure the fuel used, they measure the injector timing and calculate how much fuel should have been delivered. Differences in fuel pressure and individual injector variance mean that it only close.
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Old 04-28-2012, 05:52 AM
Bob2.8 Bob2.8 is offline
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Another thing to consider is it calculates 0mpg when stopped, but you are still burning fuel, To truly be accurate that would have to be a negative number.
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Old 04-28-2012, 07:20 AM
WestCDA WestCDA is offline
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Quote:
Another thing to consider is it calculates 0mpg when stopped, but you are still burning fuel, To truly be accurate that would have to be a negative number.
I'm pretty sure it incorporates the fuel used during that short period immediately following a reset without moving the car, when it does the averaging. If you think about it, there is no other number than zero it could display at that point - without moving the car your fuel consumption rate is 'infinite' gallons for the distance traveled. You're getting the same mileage as a stationary power plant, which is ZERO miles per gallon of fuel burned ...

If you reset the computer and drive a few blocks and it will show a preliminary figure - e.g. 14l/100 KM (on my metric setting). Stop at a light for a few minutes, and the mileage for that very short drive will slowly get worse - e.g. 14.1, then 14.2, etc. If you had not reset for several weeks, that short period of time stopped at the lights is negligible vs. the extended distance traveled, and while it becomes part of the total average it's not enough to impact the displayed value.

Last edited by WestCDA; 04-28-2012 at 07:26 AM.
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Old 04-28-2012, 09:14 AM
Bob2.8 Bob2.8 is offline
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How much it impacts the mileage depends on how mucn time is spent running the engine withouth the car moving. As an extreme if I idle for 10 gallons and drive 1 mile what is my computed mileage? We are only discussing the accuracy of the computer display right?
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Old 04-28-2012, 10:47 AM
WestCDA WestCDA is offline
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Quote:
... if I idle for 10 gallons and drive 1 mile what is my computed mileage?
Assuming US gallons, you should see .1 MPG. On my car I'd expect to see about 2352 liters per 100 KM. That's assuming it doesn't have some kind of internal cutoff for 'reasonableness'. Let us know what you see when you try that out ...

Actually, it may not even start calculating until it sees some sort of movement to begin with - so you may need to have an initial mileage reading on the computer display (following a reset) before it even kicks in. Would be an interesting experiment ... thought I'd probably idle for a little less that 10 gallons worth!

Last edited by WestCDA; 04-28-2012 at 10:56 AM.
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Old 05-28-2012, 05:58 PM
keenmachine keenmachine is offline
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How do you reset the mpg indicator? The manual is pretty vague about it.
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  #10  
Old 05-28-2012, 07:06 PM
Stromtech Stromtech is offline
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select the MPG display using the button on the stalk and then press and hold the button until it resets the display
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