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Gas mileage is suddenly 20% better!

I put on a couple of hundred miles during a highway trip recently. For most of the trip, according to the instrument panel, gas consumption in my B6 2006 VW Passat 2.0T averaged about 30-31 mpg. That’s normal for my car on the highway. Coming home the next day I stopped to refuel and, back on the highway, the gas gauge was suddenly reporting average mileage of 37 mpg. If I switched the readout from average mpg to current mpg the readout fluctuated wildly in a range of 0 mpg to 150 mpg? Can’t say I noticed any problem with performance and engine temperature remained normal. This very same scenario happened once previously. And there were other circumstances that may be related. Several weeks prior to the errors with the fuel consumption readout, the check engine light went on, a dealership diagnosed the problem as a misfire on cylinder 2. They were unable to come to any conclusion about the misfire. They did an “engine clean,” which cleared the check engine light. Word for word, it was the exact scenario in both incidents. In my opinion they have missed whatever caused the misfire on cylinder 2, and the engine clean somehow sets up the system for the fuel consumption error. Would be grateful to hear if anyone understands the problem – or the two problems!

it is not clear if you reset the meter - however that meter is operated. A “wild fluctuation”, as you say, is probably because the meter was reset - that is my experience. It is also not clear how you were driving. How one drives has a large effect on efficiency. I did not make sense of the second part of your paragraph.

also bear in mind : the function that describes miles/gallon is nonlinear, so a difference from 31 to 37 MPG is far less significant than the difference between e.g. 15 and 20 MPG:

UPDATE: after taking a walk, I understand what you are asking this all together for. Might I suggest a more accurate title, such as “gas mileage better with cylinder misfire”…

anywho, I defer to engine experts about the other dimension to your posting, but my instinct says that the mileage meter is the wrong thing to be looking at when solving a … problem … if it is one … and it is not clear that is is… about a misfiring … but again, I am no expert.

I’m Not familiar with the Passat. My 2010 Kia Forte SX automatically resets the average mpg display when it is refueled. When I accelerate to 55mph after leaving the filling station it will read 6mpg. A couple of miles later it will read 37mpg which is normal highway mpg. After leaving the highway and driving a few miles in the city it will read 27mpg which is normal city/highway combined average. I don’t know if there is anything wrong with the Passat.

If you set it to current mpg it will read 0mpg whenever you are stopped with the engine running, If you take your foot completely off the gas at highway speeds it will record 100-150 miles per gallon depending on how fast you are going, And a strong wind behind you can account for the difference between 30 and 37 if the computer was reset by fueling.
I personally don’t pay much attention to my cars mpg readings. I zero my odometers trip A setting after I fill up and divide the number of gallons into the miles travelled.

Thanks for the comments guys. I’ll keep them in mind. I’ve put more than 120k miles on the car and I am familiar with the digital data that I flip through. Plus, there is the check engine light the next day which I associate with the wacky mileage data. “Range” data when I arrived home after filling up was normal. I expect to hear about a system scan this week. Last time the scan isolated a problem with the coolant temperature sensor! Road conditions were not especially windy on the recent day. My average mileage is almost never better than 33 mpg, tail wind or no. Current consumption might hit 50 going down hill but that’s about as good as it gets. /gy

Wind does have a big effect on gas mileage the faster you travel. That would be the most logical reason for the difference.

I get much better gas mileage driving across Nebraska on I-80 going east than I do going west. Altitude drops about 3000 feet going east. Wind has a big influence too.

I don’t trust my vehicle mpg indicator. It says right now that I am getting 37mpg. A quick math check shows that it’s impossible. If my fuel tank could hold 22 gallons or so the mpg numbers might be correct but since it hold only 16 gallons at most…the numbers are bogus.

My mileage indicators have been in agreement when compared to actual fill up and mileage driven computations. You must be careful about what driving period the indicator has been reset for. Otherwise, it’s apples and oranges.