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-   -   Gas Mileage: What my TDi says, vs reality (http://forums.tdiclub.com/showthread.php?t=502862)

jadney October 21st, 2019 20:49

Gas Mileage: What my TDi says, vs reality
 
I've had 3 TDi wagons, a 2011 and 2 2015s. The 2 2015s were both purchased from a VW dealer after a Dieselgate buyback and modification.
For decades, I've kept a log of every fuel fillup, on every car I've owned since my '62 Beetle, so I could calculate gas mileage. I've always found this to be a good way to encourage good driving habits and to spot problems before they became obvious.
The diesel mileage that the 2011 calculated almost always matched the mileage I calculated, but the 2015s always display a mi/gal that is way optomistic. It's especially off at the high end. I've had a few tanks that the car tried to tell me that it was getting close to, or above, 60 mi/gal. When I run the numbers, the actual mileage is seldom above 50. On average, I'd say that the car's numbers are 5-10 mi/gal higher than actual.
I noticed this on our first 2015 and assumed it was just this car's sample variation, but that car got totaled by someone not paying attention to the road ahead of him. So we bought the second 2015. It puts out very similar numbers. It appears to me that VW is cooking the books to make it look like their modifications have actually improved the diesel mileage. It's not real.
What we're actually getting is maybe 42-45 on the highway. That's not bad, and I'm not complaining, but the 2011 proved that accurate calculations are possible and VW had the competence to do it. It appears that they have slipped up and failed to do as well with the 2015s.
We're on another long trip right now, and I just noticed another error. On the 2015s, I can look at 3 different mileage numbers: Since Start, Since Refuel, and Extended Period. If I fill up in the morning and then drive for a couple hours without stopping, the Since Start and Since Refuel should be exactly the same. They are not. I forget which is larger, but one was about 1.1 mi/gal larger than the other. That's WAY too much to be explained as a rounding error, or any other kind of excusable error.
There are clearly several problems with the calculation VW has programmed into these cars.
Has anyone else noticed this problem?

ProfBrown October 21st, 2019 21:06

Quote:

Originally Posted by jadney (Post 5545253)
I've had 3 TDi wagons, a 2011 and 2 2015s. The 2 2015s were both purchased from a VW dealer after a Dieselgate buyback and modification.

For decades, I've kept a log of every fuel fillup, on every car I've owned since my '62 Beetle, so I could calculate gas mileage. I've always found this to be a good way to encourage good driving habits and to spot problems before they became obvious.

The diesel mileage that the 2011 calculated almost always matched the mileage I calculated, but the 2015s always display a mi/gal that is way optomistic. It's especially off at the high end. I've had a few tanks that the car tried to tell me that it was getting close to, or above, 60 mi/gal. When I run the numbers, the actual mileage is seldom above 50. On average, I'd say that the car's numbers are 5-10 mi/gal higher than actual.

I noticed this on our first 2015 and assumed it was just this car's sample variation, but that car got totaled by someone not paying attention to the road ahead of him. So we bought the second 2015. It puts out very similar numbers. It appears to me that VW is cooking the books to make it look like their modifications have actually improved the diesel mileage. It's not real.

What we're actually getting is maybe 42-45 on the highway. That's not bad, and I'm not complaining, but the 2011 proved that accurate calculations are possible and VW had the competence to do it. It appears that they have slipped up and failed to do as well with the 2015s.

We're on another long trip right now, and I just noticed another error. On the 2015s, I can look at 3 different mileage numbers: Since Start, Since Refuel, and Extended Period. If I fill up in the morning and then drive for a couple hours without stopping, the Since Start and Since Refuel should be exactly the same. They are not. I forget which is larger, but one was about 1.1 mi/gal larger than the other. That's WAY too much to be explained as a rounding error, or any other kind of excusable error.

There are clearly several problems with the calculation VW has programmed into these cars.

Has anyone else noticed this problem?



The ďsince startĒ is a bs measurement. Itís not since you started the car on that run cycle, itís since you started driving within a certain amount of time. What that 1.1 miles is, is your distance from your driveway/wherever you park the car, to the gas station where you filled up. Iíve noticed it still saying ďsince startĒ even with the car sitting off in the parking lot for 30 minutes. I dont know why itís done this way, sort of misleading, but to answer your question thatís the discrepancy. This is assuming of course you drove to a gas pump before driving the rest of that morning..

For what itís worth, no car I have ever owned with an mpg reader was very accurate. Always they say a few points better than they are actually getting. My 15 GSW is about the same, only a few points off. 5 at most. I just look at the display, and guess a few points low and usually Iím spot on. My truck was usually 2 mpg points high.


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RIP TDI October 21st, 2019 21:16

MFA consumption readout on the Mk 7s is typically somewhere around 6 - 8% optimistic. The error can be reduced to in the range of 1% optimistic to 1% pessimistic using VCDS.

Your Since Start and Since Refuel reading would only be identical if you manually reset the Since Start right after you filled up.

RIP TDI October 21st, 2019 21:23

Quote:

Originally Posted by ProfBrown (Post 5545259)
The “since start” is a bs measurement. It’s not since you started the car on that run cycle, it’s since you started driving within a certain amount of time. What that 1.1 miles is, is your distance from your driveway/wherever you park the car, to the gas station where you filled up. I’ve noticed it still saying “since start” even with the car sitting off in the parking lot for 30 minutes. I dont know why it’s done this way, sort of misleading

VW MFAs have had the Since Start value forever and its legit. It automatically resets after sitting off for 2+ hours, but runs continuously up to a large max value as long as the car is running, including off periods of less than 2 hours between running periods.

ProfBrown October 21st, 2019 21:25

Quote:

Originally Posted by RIP TDI (Post 5545262)
VW MFAs have had the Since Start value forever and its legit. It resets after sitting off for 2+ hours, but runs indefinitely as long as the car is running, including off periods of less than 2 hours between running periods.



Couple points I should make, this is my first VW that is newer than 1973, and second my claim that itís ďBSĒ is due to it being called ďSince StartĒ when itís not per start. Itís essentially per driving leg of a journey, apparently if the stop is less than 2 hours the car reads as if it never stopped which is counterintuitive to the name.

Makes more sense now with your clarifying post, and for that I thank you. But from a newbie to the car thatís why I said it was BS LMAO.


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jadney October 22nd, 2019 05:19

Quote:

Originally Posted by RIP TDI (Post 5545261)
MFA consumption readout on the Mk 7s is typically somewhere around 6 - 8% optimistic. The error can be reduced to in the range of 1% optimistic to 1% pessimistic using VCDS.

What I see, on both of the 2015s that I've had is an error of more like 20%. That's when the car says 60 mi/gal but the actual number comes out to 50.

Okay, revealing my ignorance, what is VCDS?

oilhammer October 22nd, 2019 05:52

I have found over the years that the MFA's fuel economy trends closer to accurate the longer the trip is. So for shorter trips (less than 100 miles) it trends to 5-8% higher than actual, but for a 500 mile trip it ends up closer to 2-3% accurate. But it is always on the side of optimistic. Plus, it is going by the fuel measured by the ECU as being injected into the engine, not by fuel gauge position, although gauge position is what the MFA uses to calculate "distance to refueling". Which I always get a kick out of when you start out with a full, vented tank, because THAT value goes UP for the first ~50-100 miles. :p

IndigoBlueWagon October 22nd, 2019 07:49

I adjusted the FE readout in the cluster in my '15 GSW using VCDS to 7% lower than it was reading. Pretty dead on now, maybe 1 MPG low for a full tank. There are instructions here somewhere on how to do it.

My BMW consistently reads 2-3 MPG low. And to the best of my knowledge I can't adjust it.

Honestly, I find the cluster FE readings annoying. I prefer to have a rough idea of how I'm doing by reading the fuel gauge, and then calculating FE when refilling. Less stressful.

KERMA October 22nd, 2019 08:01

in b4 "your car doesn't use gas so no gas mileage" :D

the accepted verbiage around here is "fuel economy" FWIW

RIP TDI October 22nd, 2019 08:19

Quote:

Originally Posted by ProfBrown (Post 5545263)
Couple points I should make, this is my first VW that is newer than 1973, and second my claim that itís ďBSĒ is due to it being called ďSince StartĒ when it’s not per start. Itís essentially per driving leg of a journey, apparently if the stop is less than 2 hours the car reads as if it never stopped which is counterintuitive to the name.

Makes more sense now with your clarifying post, and for that I thank you. But from a newbie to the car thatís why I said it was BS LMAO.


Sent from my iPhone using Tapatalk

I take your point. On the older A3/B4/A4s, it was called "Trip 1" instead of "Since Start" and "Trip 2" instead of "Extended Period". Less misleading in some ways, more so in others.

RIP TDI October 22nd, 2019 08:25

Quote:

Originally Posted by jadney (Post 5545285)
Okay, revealing my ignorance, what is VCDS?

Get used to searching here; there's a wealth of info on this site.

1854sailor October 22nd, 2019 08:32

Quote:

Originally Posted by jadney (Post 5545285)
...Okay, revealing my ignorance, what is VCDS?

You've been around this site since 2014 and you've never heard of VCDS??

RIP TDI October 22nd, 2019 08:47

Quote:

Originally Posted by oilhammer (Post 5545304)
I have found over the years that the MFA's fuel economy trends closer to accurate the longer the trip is. So for shorter trips (less than 100 miles) it trends to 5-8% higher than actual, but for a 500 mile trip it ends up closer to 2-3% accurate. But it is always on the side of optimistic. Plus, it is going by the fuel measured by the ECU as being injected into the engine, not by fuel gauge position, although gauge position is what the MFA uses to calculate "distance to refueling". Which I always get a kick out of when you start out with a full, vented tank, because THAT value goes UP for the first ~50-100 miles. :p

Well, of course the ECU infers fuel flow from a variety of data rather than actually measures it, and that is the source of the non-linear error you mention.

My understanding is that "distance to refueling" uses inferred accumulated fuel used rather than gauge position for calculation, at least on the Mk 7. And that also explains the phenomenon you describe. The beginning value after a fill up is based on the calculated value from the previous tank and gets adjusted by inferred current fuel flow rate, which can be lower or higher than the average rate of the previous tank.

RIP TDI October 22nd, 2019 08:51

Quote:

Originally Posted by 1854sailor (Post 5545338)
You've been around this site since 2014 and you've never heard of VCDS??

Ha, I knew someone would take it a step further than I did and should have guessed it would be you, Sailor :D

IndigoBlueWagon October 22nd, 2019 08:57

Maybe not apropos of much in this discussion, but I took this photo this AM because I liked the even numbers of fuel used and remaining.
http://pics.tdiclub.com/data/500/medium/200.jpg
It also shows I haven't driven the car much in the two and a half year's I've owned it.


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