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Why Small Turbo Engines Get Lousy Mileage

The Fed

Old Guys Rule
Location
Florida
Since the diameter of the tire varies with tire pressure, and all of the MPG calculations are probably based on the left front wheel speed sensor, that can explain a few MPG difference if the tire is not at whatever PSI is stated on the door jamb. That would make the on board computer virually useless. You're two pounds low, your tire is very hot, there could be 5% right there. Your tire is half worn, it probably reads even less. And where does the car pick up fuel consumed? I'm not aware of a measuring flow sensor, and we don't have MAF's, right? So I figure everyone is right.
 

heiney9

Go Kart Champion
Location
Illinois
Car(s)
2017 GTi Sport DSG
I did not know that. I will check mine...I assumed it would be more accurate that the old manual fill up/check miles/gallons method as consistent fillup levels introduce an element of error. Here in ORegon we can't pump our own gas, so I just have to hope the fill up is consistent each time...to me that seems like a big variable that can skew results.


On the other hand WTF is it with VW intentionally programing that mileage computer to give erroneous (optimisitic) MPG figures...that totally sucks.


But then on the other hand i could care less if i actually got x MPG vs y, i just got kinda sucked into this discussion with naysayers saying x mpg city is not possible whereas my computer is telling a certain story that may be alternate facts??


I check to see how my computer reads relative to manual calcs and post at a later date.


The on board computer, since it was invented, has always been an average over a certain distance in time. If you know anything at all about those words as a function of mathematics, you'd understand why it varies so much. The longer time you average, the more accurate the calculation. It has ZERO to do with anyone intentionally programming something to give false readings.

It's meant to be a guide, nothing more, nothing less. Just like your fuel gauge is a guide.
 

heiney9

Go Kart Champion
Location
Illinois
Car(s)
2017 GTi Sport DSG
I'm sure they use an algorithm to calculate fuel usage. If you baby it, the readings are higher, if you floor it, the readings are lower. To get the most accurate information is to allow the average to be calculated over a longer period of time, like a full tank of gas vs. a quick run to the store.
 

Strange Mud

Autocross Champion
Location
Small Town CT
Car(s)
Assorted
I have read from a few reviewers that the GTI is one of the few cars that they can get close to/better than the EPA numbers.
 

danall

Passed Driver's Ed
Location
USA
The on board computer, since it was invented, has always been an average over a certain distance in time. If you know anything at all about those words as a function of mathematics, you'd understand why it varies so much. The longer time you average, the more accurate the calculation.

I'm totally aware of that but it doesn't explain why some folks are saying it is always overexaggerating fuel mileage, rather than sometimes under and sometimes over. Anyway i calculated my city driving on 229 miles of exclusively city driving over a two week period, GTI computer had it at 30.1 MPG while i manually calculated it based on fillup volume (which easily can be a variable) at 29.87 MPG. Yes i could try running close to a full tank to check but i think a couple hundred miles should be enough to give a good approximation. I'm also aware that if i check this during winter months it would be much different. All i can really say is that without specialized equipment you can never get really totally accurate MPG figures but i figure the GTIs on board computer (or the manual method) over at least a couple hundred miles should be close enough...it is for me. So I do stand corrected, using the manual calc method i only got 29.87 not 30MPG.

Now i could try to get his average over a much longer period of time and distance, but the problem with that scenario is that there occasions when i do leave the city limits and do rural driving so that is out. (usually use my motorcycle or street rod for non-urban driving/pleasure cruising).
 

Strange Mud

Autocross Champion
Location
Small Town CT
Car(s)
Assorted
I'm totally aware of that but it doesn't explain why some folks are saying it is always overexaggerating fuel mileage, rather than sometimes under and sometimes over.
for me it's because only once did it under report mpg. in the 10 months/10K miles I've owned it.. I've done math on 98% of my fill-up. I know changing the display won't change my real mpg but OCD and ODBeleven are changing that for me.
 
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