I have a 2000 A6 2.7t. I have had the car for a couple months and have been working on the general maintenance to get it up to date. I drive an average of 75 mph and predominately on highways. I understand that these cars are not designed for great gas mileage, but the sticker says 17(city) 24(hway).
My gas mileage on the first tanks of gas were around 16 mpg. Then after performing the maintenance i saw the mpg increase. I changed synthetic oil, fuel filter, air filter and diverter valves. The mileage seemed to increase by 2-3 to around an average of 18mpg. Now lately the mileage has fell back down. With as much Highway, non-stop driving i do i think it should be consistently closer to 20mpg.


My fillups over the past couple months have given these values,
14.8 --> 18.1--> 19.6--> 21.1--> 15.2--> 17.6--> 13.9--> 18.9--> 17.6 (mpg)
(I have been driving back and forth between chicago and toledo for the holdiays, so the last few tanks have been 90% highway driving on cruise control.)

The only thing i have left to do is change the spark plugs, but i dont think that would cause the great fluctuation in values that i have had. I am really at loss for what the cause could be.

Any ideas from all the experts here?

Thanks,
Aaron

 

You're doing all the right things. Bottom line is you're driving a car that weighs almost 2 tons and has AWD. I suggest you check tire pressure and consider running it at 40psi. I used to run mine at 36psi but Belle Tire had no objections to running them at 40.

For comparison, I bought mine with 37k and now have 159k. I keep a log of repairs and gas mileage -- calculated, not the optimistic car computer. I mostly drive highway and see 22-23mpg when driving ~75, pushing 24-25mpg if I have the patience to drive 60-65. Around town it seems to run 18-20, but I do little city driving.

I replaced spark plugs at 100k even though my indie shop pulled one and declared them fine. My gas mileage did not change with the new plugs and I have postponed replacing them again even though they now have almost 60k on them. I also installed a K&N air filter years ago and did not notice either an improvement in gas mileage nor power.

I live in Michigan so we share the same delightful climate. I notice my gas mileage decreases when the temperature is below 30 vs above 70. I don't know if this is due to the car adjusting it's own mixture or different characteristics of the gasoline (e.g., some kind of winter blend). But the same route I drive (Holland to Detroit and back) in the fall or spring yields 23-24 mpg and in the winter it is 21-22 mpg. When the temperature is 70-80 I get 400-410 miles per tank, but in the winter it is barely 375. And I drive slower in the winter due to road conditions. The last variable is sometimes I fill up with 10% ethanol (not by choice) and it does not give as good gas mileage as pure gasoline. Oh...and I use midrange 89 octane. I have tried a few tanks of premium 91-93 and found no difference in gas mileage.

 

Thanks alot for your take on things. It seems like you have the numbers i would be happy with. I drive about 40 miles each way to and from work (around chicago) during the week and i have been getting less than 300 per tank. I realize that isnt alot lower but that is 100 miles more i could have between fill-ups. And i have been using 93 octane BP gasolive. I might switch over to the lower octane if you say that you notice no difference.

I will check the tire pressure when i get home.

Is there anything else i could replace or clean or update that might contribute to the sub-par fuel economy?

And also, my values were calculated from teh actual miles driven divided by the gallons of gas used to fill the tank completely.

 

I don't think you are supposed to run a lower octane fuel in these cars. I ran some low octane in my 2.8 and noticed it ran a bit more rough. One thing pick-up truck guys do is block off their upper grille and they say it yields better fuel economy. I think this may be safe for the winter due to the low temps and the lower grille's that will still allow for air flow. Wouldn't want to try it in the summer though. Also you have to be careful about over inflating the tires, it will wear out the centers of the tires faster but it is a good way to get better fuel economy. I just put on snow's and saw a HUGE drop in my economy (almost 2 mpg); needless to say tires have a very large impact on mpg's. I skinnier tire and harder compound will help with mpg's along with lighter wheels. The best way to combat bad fuel economy is a 4 banger beater for work and a nice car for leisurely driving (I have a sentra that gets 32 mpg combined hwy and city and I picked it up for dirt cheep).

 

My daughters 1998 Audi A6 wagon 2.8L 12 valve engine was bought used and went more than a year before the head gaskets failed. The exhaust had water coming out of the tail pipe. Also, the car would overheat when in traffic jambs. It also got terrible mileage.
The engine block coolant drain was competely plugged by black cooling system anti-leak compound. Since it was plugged up behind the rear crankshaft seal flange, I had to remove the engine. With the cylinder heads off, this was not very difficult, since it was a C4 chassis Audi. After new head gaskets, new valve stem seals, new timing belt, etc., the engine got better performance and mileage.
You could check each of the sparkplugs for fouling. Change the fuel filter. Check the compression of each cylinder. However, do the cheapest first, which would be to take the car to an auto store which provides free OBDII code reading.
Old transmission fluid and gear oil, may also affect mileage.

 

There is always factors in our sweet climate up here that increase gas mileage. For me, the big one would be letting the car heat up in the morning. I dont like to drive it till its nice and toasty. Not only for comfort, but getting your oil up to temp is important in order to avoid unnecessary engine wear.

Another big thing is all the extra weight from snow and ice build up, which can add up to 100lbs to your vehicle. This can reduce your mileage by up to 2%!

Check your tire pressure. If you dont run nitrogen, every 5 degrees Celsius the temperature drops, the pressure in a tire will drop by one pound per square foot. I run mine at 40psi year round (on my 255/45s) anything lower than that I find the car to be too sloppy.

Winter Gasoline blends will also affect your MPG. Dont run ethanol, its horseshit.

Oh, also running defrost runs your AC

 

If I'm cruising(I'm in Michigan also) I get around 25mpg at 75mph, thats calculated.

Now my tank average is around 19 because its tuned and makes 17lbs of boost, I also have a manual.

You will get worse gas mileage in the winter because there is less energy in the fuel, about 5 percent less, so it takes 5 percent more fuel to make the same power. BTW, Ethanol is mandated in the United States now, 10 percent, governments gotta keep the corn farmers happy.

If I drive it hard I still can't get it below 15mpg. On the freeway I get better mileage than my wife's 3.0 A4.

Tires do make a big difference. My wife's A4 gets around 22 until we put the winter tires on, winter tires+winter gas is about a 12 percent drop in mileage. I have the OEM Pirelli's.

 

So from reading all of your responses, it seems like youre saying i just need to find the right combination of driving habits and tire pressure. I can pull the plugs just to check them, but if one was fouled i would notice a difference in performance wouldnt i? What should i be looking for in the plug? I usually just change them and dont worry about it, but the plugs that i found recommended for this car (NGK pfr6q) are about $15 each so i'd like to be sure its necessary.

Eric C,
You say yours is tuned, do you have an APR or GIAC chip? Do those chips increase or decrease mpg?

 

I havent changed plugs in this car yet, but the stock delco plugs for my STS are also very expensive. I have put "premium" platinum plugs into it before to save some money. Big mistake. Ran like **** very quickly. I assume these audi cars are the same.

Its hard to tell via the butt dyno a loss of a couple HP from bad plugs. Unless it clearly missfires. (put it in 3rd at low rpm and speed then give it gas. Usually if there is a missfire, it will show up under load at low rpm)

 

If your in Michigan goto VAST in Ferndale.

I had VAST tune my car, they started off with a base tune then did some short drive ability tuning so its more custom. Same price as the other vendors and they can make changes on the fly right there.

For the cost they also do a quick once over on the car as far as boost leaks and that. Been going there since I bought the car and always been happy with them.

Plus my car is pretty zippy in comparisons to other 2.7's. I've had B5 S4 owners in my car asking how much work was done to it, but I just have the tune in it.

Tuning the car will decrease mileage if you keep your foot in it but they also can lean it out a bit off boost to increase mileage.

Remember engine tunes are for 50 states so for people who live in the desert to the people who live in the snowy mountains. The best way to cool an engine is to keep it rich so if your mostly sticking to your climate area then you can lean it out a bit and make some more power and increase mileage.

I also have the expensive Audi recommended plugs, put them in to fix a misfire issue at idle that never has gone away but I'm suspecting a bad coil and will replace those next.