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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old December 15th, 2008, 16:39   #1
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Join Date: May 2005
Location: penna, usa
Fuel Economy: ?
Default 2009 TDI mileage monitering system

I have a 2006 and am considering a 2009. I am wondering how most of you on this forum determine mileage stats. The computer in my car is a joke and I'm wondering how many people here who posted mileage have quoted the computer or have "emptied" the tank? Basically since I can't trust VW I would like to know what to expect to get mileage wise. Also, why can't VW make a computer that really measures mileage accurately? Jim
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Old December 15th, 2008, 16:46   #2
Plus 3 Golfer
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Join Date: Oct 2008
Location: ARIZONA

Here's my hand calculated mpg. I top off so that I can see the fuel each time. The MFD MPG is very accurate when compared to my hand calculated value. I don't recall the MFD MPG to be more than 0.2 MPG different from the hand calculated number for each tank. On a 5900 mile trip from AZ to VA and back, the MFD MPG showed 41.0 MPG and the hand calculated was 40.86.

Last edited by Plus 3 Golfer; December 15th, 2008 at 16:53.
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Old December 15th, 2008, 17:14   #3
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Join Date: Aug 2008
Location: Southeastern US

Originally Posted by bostelle
Also, why can't VW make a computer that really measures mileage accurately? Jim
They can, and they do; my MFD is within +/- 0.2 MPG of hand-calculated values, others have reported the same, and no one I can recall has reported more than a 1 MPG difference (to put that in perspective, that latter value would be about a 2.5% error...pretty trivial, and to my mind that small an error might be attributable to either the ECU or the hand-calculator...for example, not every filling station pad is level).

In years past, the ECU had no way of actually knowing how much fuel was delivered with each pop of the injector, so it just used a generic value...hence the large errors on older cars. On the '09 TDI, the ECU critically needs to know how much fuel is injected, so there are pressure sensors in the glow plugs, plus other stuff, which allow an accurate determination of fuel injected. I forget all the details; Drivbiwire has discussed this, as I recall. Hence the '09 MFD display of MPG is pretty durn accurate. I double-check mine about every tank or two, and it is always spot-on.
2009 Jetta TDI (Loyal Edition, Campi White, DSG, #1470, built 5/08): 56K miles and zero problems 1/16/2011.
WeatherTech deflectors, Dieselgeek skid plate (both A+)
My other diesel gets 120 MPG (minutes per gallon): Kubota 2630 w/ FEL
My other other diesel I don't even want to talk about (1995 Dodge Ram 2500 Cummins 4x4)
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Old December 15th, 2008, 19:12   #4
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Join Date: Jul 2008
Location: Aurora, OH
TDI(s): 09 Jetta TDI

With 14K on my 09, I have hand calculated every tank, and compared to MFD. I have had one instance of variance by 1.5 mpg... otherwise it consistently was off around 1/2 mile - until the weather changed. My most recent two tanks found that it underated my mpg around .5 mpg. But all said and done, the mfd is very close.

09 Jetta TDI, Campanella White, DSG Loyal
Aurora, OH
Mileage 42.5 to 50 mpg (when I REALLY try)
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Old December 15th, 2008, 19:28   #5
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Location: Philadelphia, PA

I've only filled up twice thus far (third one before the end of this week) and both tanks, the MFD reported an average mpg that was slightly LESS than my hand-calculated values.
"I interviewed the late Steve Yokich, then UAW president, almost ten years ago. When I suggested that Detriot might be putting too many eggs in one basket with its utter devotion to SUVs at the expense of passenger car development and asked if the union might take a position in favor of more economical cars, he started screaming at me." -Jamie Kitman, Automobile
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