On our recently new to us 2000 A6 4.2 Quattro the instant MPG readout, and the average MPG seems abnormally high, averaging around 31 mpg. Often driving on a level road, we will see instant readings of high 30-40mpg.
This is not anywhere near what our milage is by keeping track of it.

I do have error code 17538 & 17536 showing up after about 40 miles which I attribute to a vacume leak. I've replaced all the small vacume hoses, which made it not return quite so fast, but there must still be one hiding somewhere.

My question is, could the vacume leak be the cause of the erronious readings on the average & instant MPG shown in the information display?

Thanks,
Ken

 

I have noticed it on my 3.0 when I forgot to connect a line.

A fresh leak might cause it to get better mileage but the ecm will soon recalibrate the fuel to air ratio and it will go back down.

I don't believe the readings anyway but If mine had a vac leak I would repair it immediately.

 

thanks for the reply Jack. That was the jist of my poorly worded question I suppose - are the readings a product of the ECM, or are the readings shown like the old vacume/fuel economy guages.

I agree about getting the leak fixed. Yesterday I replaced about 8' of the small vacume hoses under the engine covers. The old cloth-wrapped hoses were in horrible condition. After replacing them I cleared the two faults with the Vag-Com but they both returned after about 30 miles which was a huge improvement. Previously it only took about 2-3 miles for them to return which tells me that I'm on the right track chasing vacume leaks. (whew)

You stated that you don't believe the readings. Do you not find the average readout to be fairly accurate?

Thanks,
Ken

 

If I remember my owner's manual, the instant mpg is calculated every 6 seconds. If you are accelerating, it is low; if you are driving steady speed, it is roughly accurate; if you are decelerating, it is high. Resetting the average mpg with each fill up will give you a more accurate reading, but I have always found my average mpg reading to be 1-2 mpg optimistic (higher than when I calculate miles driven/gallons filled). I use my A6 for mostly highway driving and the average is typically 23-25 mpg while the calculated is more like 22-23. I also drive 70-75. If I have the patience to drive ~60 I see closer to 24-25 actual mpg. But I don't...have the patience.