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Old July 20th, 2012, 20:58   #1
kgall
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Default How much diesel left when the display says 0 miles left? And accuracy of MPG calcs.

On an Audi A-3 TDI,
how much fuel is left when the display says you have 0 miles left before refueling?

On my Prius, the answer is generally agreed to be 1.9 gallons or so.


Also, how accurate is the MPG calculator generally thought to be on Audi A-3s?
Is there any way to keep the MPG calculator from resetting if the car has been off for two or more hours--I would like to be able to keep it going for full tanks, so I measure the error for myself.

Again, on my Prius, the computer reads about 5%-7% too high, based both on my experience and the consensus on a Prius fan website like this one.
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Old July 21st, 2012, 05:01   #2
Birdman
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the Audi works like the VW TDI the MPG display has 2 settings, 1 that resets if the car has been off for 2 hours and one that will hold the settings till you reset it. On my 03 Jetta there is a button on the bottom of control stalk that switches between the 2 settings. You might have to look in your manal if you have one or check Google.

My car on the miles to empty, the low fuel light will come on at 80 miles till empty. I recalibrated my tank so that the light comes on with 2 gallons of fuel left. So it is fairly accurate. when my MTE show 40 miles I will put 15.5 to 16 gallons in the tank.
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Old July 21st, 2012, 08:20   #3
kgall
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Birdman,
Thanks, I will look for the "second computer".
My low fuel light comes on at 50 miles till "empty". The fuel gauge shows about 1/8 of a tank left.
I guess my question is, how much is left when the gauge and/or the "low fuel" warning says "empty"?
Every car I've ever had still has fuel at that point, and I'm just wondering how much.
I'm the sort of idiot who likes to know that stuff, but not the sort of idiot who deliberately runs my car out of fuel just to find out.
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Old July 21st, 2012, 08:52   #4
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I would not chance running a CR out of fuel.

This question about calculated mpg accuracy comes up from time to time and I always wonder why people doubt the accuracy on modern vehicles. The fuel injection system is so precise that the fuel use can be very accurately tracked. Why would they be off?
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Old July 21st, 2012, 12:47   #5
Birdman
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Your manual will have all those answers, so you really need to find it. I just guess that your car is mostly the same as our VW's if that is the case when the light comes on the book IIRR says 1.9 gallons left. I like to know also, I opened the tank pumped all the fuel out , then added 2 gallons back. Then I used the VCDS and made the fuel gage read right on the red mark where the light come on.
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Ventectomy,Alientech window,Old Navy ccv filter,Catch can,Dieselgeek short shifter,ATE power slot rotors,Porterfield pads,Shine suspension,Shine rear sway bar,TT pedals & foot rest,Cat fuel filter 2 micron,Alum skid plate,Euro headlights and switch,5 Brake light mod.
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Old July 21st, 2012, 12:54   #6
kgall
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Quote:
Originally Posted by tditom View Post
I would not chance running a CR out of fuel.

This question about calculated mpg accuracy comes up from time to time and I always wonder why people doubt the accuracy on modern vehicles. The fuel injection system is so precise that the fuel use can be very accurately tracked. Why would they be off?
Who knows why?--my guess is to make folks feel good. I know that in the Prius the computer shows MPG about 5 to 7% higher than you actually get--one of a few things that annoy me about the car--because you are right that there's no reason they shouldn't be accurate.
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Old July 21st, 2012, 13:01   #7
Roshermoore
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Quote:
Originally Posted by kgall View Post
On an Audi A-3 TDI,
how much fuel is left when the display says you have 0 miles left before refueling?

Also, how accurate is the MPG calculator generally thought to be on Audi A-3s?

You know there is an A3 Discussion Area down the page in the listing of Discussion Areas.
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Old July 21st, 2012, 13:14   #8
tditom
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Quote:
Originally Posted by kgall View Post
Who knows why?--my guess is to make folks feel good. I know that in the Prius the computer shows MPG about 5 to 7% higher than you actually get--one of a few things that annoy me about the car--because you are right that there's no reason they shouldn't be accurate.
but why are you so confident the computer calculated numbers in your prius are wrong?
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Old July 21st, 2012, 13:23   #9
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Remember that the fuel in the tank keeps your in-tank fuel pump cool and lubricated. So when your sucking in the last of the fuel at the bottom of the tank your pump is not being cooled by the surrounding fuel and your ingesting all of of the swill at the lowest point of the fuel tank.
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Old July 21st, 2012, 14:54   #10
kgall
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Default Errors, Running out of Fuel, the A-3 Forum

Quote:
Originally Posted by tditom View Post
but why are you so confident the computer calculated numbers in your prius are wrong?
Thanks to all

tditom, for every single tank of fuel I've put in my Prius and measured the computer against my calculation of miles driven and fuel used, the computer has read about 1.9-2.7 mpg too high. For the dozens of tanks, it averages out to about 2.4 mpg too high. The variation in error for each tank could be due to errors in how much I fill it up each time. The consistency of the errors, I don't think so.

For 03Springer and others who think I would be an idiot to deliberately run out my tank, I agree for all the reasons you give. I like to know how far I can go before running into those dangers.

Roshermoore, I've only been on this list a few weeks. But I've noticed that there are a lot more people reading this forum than the A-3 forum, and I've gotten a lot more useful answers when I've posted here, even if sometimes things are not exactly the same for VW and Audi TDI's.
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Old July 21st, 2012, 15:06   #11
tditom
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So you think that the odds that the computer is wrong are better than your filling technique being the big variable?

If you can't be assured of a top off to the same level each time, then you really don't know how full your tank is when the pump switches off.
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Old July 21st, 2012, 15:29   #12
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Quote:
Originally Posted by tditom View Post
So you think that the odds that the computer is wrong are better than your filling technique being the big variable?
Yes?

Now, if it was off 1.5-2 mpg, sometimes high, sometimes low, then yea I'd say variable. But when you get the same offset tank after tank after tank, it is much more likely to be the computer misreporting than operator error.

Read the Mk6 Golf forums some time. Our cars do the EXACT same thing, with most repeating the the computer MPG is 1.5-2MPG high consistently.
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Old July 21st, 2012, 15:39   #13
tditom
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OK- so this would be intentional mis-programming if true.

I will keep a closer eye on my Camry Hybrid consumption (hand calc vs computer)
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Old July 21st, 2012, 16:18   #14
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My pet theory goes like this - Our speedo is set to read a bit high, and it's THIS number that is used to determine instantaneous MPG, and that tank MPG is a continuously averaged derivative of that. So basically the built in speedo error is what causes it to read high.
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Old July 21st, 2012, 16:43   #15
tditom
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so they back calculate the distance by speedo reading, instead of using the odo?"
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