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Discussion Starter · #1 ·
I have the extended range fuel tank, which is 16.9 US gallons. With a full tank, the range starts off at somewhere around 430 miles, which makes sense as, according to the car's trip computer, I have been getting 26 mpg mixed city/highway. The re-fuel indicator came on at about 350 miles and I filled up at about 380 miles. I expected to put in about 14.5 gallons, but it took 15.5 gallons to fill up the tank. That means that my actual MPG was 24.5, a drop of 1.5 gallons, which I consider to be materially different from that shown by the trip computer.

Has anybody noticed a similar difference? What could account for the actual MPG being so different from the trip computer's calculation?

Also, about how many gallons are left in the tank when the re-fuel indicator comes on?

Thanks.
 

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Discussion Starter · #3 ·
Thanks for pointing that out.
 

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My fuel light comes on at 50 miles to go.

My average fuel consumption according to the computer is currently 19.1 mpg. I'd say that is pretty much dead on as I fill up after about 270-280 miles. And I need to put in around 15 gallons.
 

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Discussion Starter · #5 ·
I just put in my third tank of gas. Just like the previous one, the miles per gallon indicator in the trip computer was off by 2 miles per gallon (the trip computer showed 26.3 but the amount of gas I had to put in calculates to 24.3). I have had cars before when the trip computer was off by 0.3 to 0.5 miles per gallon, but nothing ever this large. I don't understand how it can be that far off, and I am wondering if it points to an issue with the engine operation or software controlling the engine. And this isn't just my OCD... the fuel gauge said I had 24 miles left, but since I put in 16.4 gallons, I only had 12 miles left until I ran out of gas.

If any of you have any suggestions, I would love to hear it. I am going to be raising this with my SA next week. I will let you know what he says.
 

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Mpg, real

I have had this MPG discrepancy in every car that I have owned that had a computer measuring MPG. Either the manufacturers are cheating( imagine that ) or the service stations are cheating.
In terms of relative MPG, my 718CS is very sensitive to my driving habits. I hate the start stop, normal, auto mode and I'm sure that I sacrifice mileage. BFD
I drive my car in manual and non-sport on trips and get decent MPG. If I put it in sport mode, my MPG suffers.
But back to the computer measurement, I just accept that someone is lying. I like my '17CS so much that I just don't want to let this upset my ownership of this car.
 

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Discussion Starter · #7 ·
Update for those that are interested:

Just got off the phone with a service technician at my local (Woodland Hills) Porsche dealership. He said that the trip computer was just an estimate of MPG and MTE (miles till empty), which, of course, we knew. The thing is that each time I have filled up, my trip computer has been exactly 2 MPG off. So, whether or not it is an estimate, it follows a set algorithm and I believe that something is wrong.

More frustrating is the fact that last time I filled up the, MTE said I had 24 miles left when I actually only had 12. Trip computers are set up to be conservative, saying you have less MTE than actual so you don't wind up driving the car to empty. In fact, in every car that I have owned with a trip computer (going back to my 1988 GTI), when MTE said "0", I still had at least one gallon of gas left in the tank. The fact that the MTE was so wrong does concern me.

The service technician said that the differences I was seeing in MPG and MTE were higher than anything that he had encountered before, so he would have to look into it and get back to me.
 
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