Gas Mileage: What my TDi says, vs reality

jadney

Member
Joined
Nov 9, 2014
Location
Madison, WI
TDI
2015 Golf Sportwagon TDi DSG
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

Veteran Member
Joined
Jan 16, 2019
Location
So Cal
TDI
2015 GSW SE DSG
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 “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, but to answer your question that’s the discrepancy. This is assuming of course you drove to a gas pump before driving the rest of that morning..

For what it’s 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 I’m spot on. My truck was usually 2 mpg points high.


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RIP TDI

Top Post Dawg
Joined
Feb 16, 2000
Location
Santa Barbara, CA
TDI
'15 GSW SE 6MT, '01 Golf 5MT, '97 Passat sedan, '96 Passat variant
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

Top Post Dawg
Joined
Feb 16, 2000
Location
Santa Barbara, CA
TDI
'15 GSW SE 6MT, '01 Golf 5MT, '97 Passat sedan, '96 Passat variant
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

Veteran Member
Joined
Jan 16, 2019
Location
So Cal
TDI
2015 GSW SE DSG
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 it’s “BS” is due to it being called “Since Start” when it’s not per start. It’s 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 that’s why I said it was BS LMAO.


Sent from my iPhone using Tapatalk
 

jadney

Member
Joined
Nov 9, 2014
Location
Madison, WI
TDI
2015 Golf Sportwagon TDi DSG
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

Certified Volkswagen Nut & Vendor
Joined
Dec 11, 2001
Location
outside St Louis (where it's safe)
TDI
There are just too many to list....
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

TDIClub Enthusiast, Principal IDParts, Vendor , w/
Joined
Aug 16, 2004
Location
South of Boston
TDI
'97 Passat, '99.5 Golf, '02 Jetta Wagon, '15 GSW
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

Vendor , w/Business number
Joined
Sep 23, 2001
Location
here
TDI
99 beetle and 04 jetta
in b4 "your car doesn't use gas so no gas mileage" :D

the accepted verbiage around here is "fuel economy" FWIW
 

RIP TDI

Top Post Dawg
Joined
Feb 16, 2000
Location
Santa Barbara, CA
TDI
'15 GSW SE 6MT, '01 Golf 5MT, '97 Passat sedan, '96 Passat variant
Couple points I should make, this is my first VW that is newer than 1973, and second my claim that it’s “BS” is due to it being called “Since Start” when it’s not per start. It’s 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 that’s 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

Top Post Dawg
Joined
Feb 16, 2000
Location
Santa Barbara, CA
TDI
'15 GSW SE 6MT, '01 Golf 5MT, '97 Passat sedan, '96 Passat variant
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.
 

IndigoBlueWagon

TDIClub Enthusiast, Principal IDParts, Vendor , w/
Joined
Aug 16, 2004
Location
South of Boston
TDI
'97 Passat, '99.5 Golf, '02 Jetta Wagon, '15 GSW
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.

It also shows I haven't driven the car much in the two and a half year's I've owned it.
 

RIP TDI

Top Post Dawg
Joined
Feb 16, 2000
Location
Santa Barbara, CA
TDI
'15 GSW SE 6MT, '01 Golf 5MT, '97 Passat sedan, '96 Passat variant
It also shows I haven't driven the car much in the two and a half year's I've owned it.
And I thought I was the king of the low odometer value. Peter, you've humbled and shamed me! I can no longer say, "I drive so little and such short distances, I should get an EV."
 

IndigoBlueWagon

TDIClub Enthusiast, Principal IDParts, Vendor , w/
Joined
Aug 16, 2004
Location
South of Boston
TDI
'97 Passat, '99.5 Golf, '02 Jetta Wagon, '15 GSW
It's because I can't stop driving this one:

That was trip tank coming east from Wisconsin to MA a couple weeks ago. Headed to NC in that car tonight.
 

turbobrick240

Top Post Dawg
Joined
Nov 18, 2014
Location
maine
TDI
2011 vw golf tdi(gone to greener pastures), 2001 ford f250 powerstroke
Nice. You should give IBW a good wash/detail when she turns over 400k. That's a loyal car.
 

jadney

Member
Joined
Nov 9, 2014
Location
Madison, WI
TDI
2015 Golf Sportwagon TDi DSG
You've been around this site since 2014 and you've never heard of VCDS??
I signed up when we bought the 2011, but never came here much because the car ran so well, so I never had any questions.

Okay, I'll search for VCDS, but I suspect it will show up alone most of the time, without explanation.
 

jadney

Member
Joined
Nov 9, 2014
Location
Madison, WI
TDI
2015 Golf Sportwagon TDi DSG
You've been around this site since 2014 and you've never heard of VCDS??
I signed up when we bought the 2011, but never came here much because the car ran so well, so I never had any questions.

Okay, I'll search for VCDS, but I suspect it will show up alone most of the time, without explanation. Okay, got it: VAG-COM Diagostic System (or Software.)
 

sprstu

Veteran Member
Joined
Sep 6, 2005
Location
Minneapolis, MN
TDI
mk1 TD, GSW TDI 6spd
Yep, the at home diagnostic tool that allows users to go into the cars programming and change things or take measurements.

My current screen for FE. The grey line is what the MFA says and the actual line is calculated. The problem got worse when I got my Kerma Tune, I just figured the ecu was being fooled by the tune and the calculations were based on a stock formula. Pre-tune the numbers where much closer to reality.

https://imgur.com/gallery/gVCw6Vi
 
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jerrymander

Veteran Member
Joined
Feb 11, 2012
Location
Cowford, FL
TDI
2011 Golf Reborn Edition, 2015 Golf, 2011 Golf w/ Factory Crushed Option
my 2015 is 10% optimistic exactly. I tired to fix it with VCDS but couldn't really get any changes to display properly
 
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