I will admit that I suck at math, but their fuel consumption calculations make me feel like Einstein...

So I have a few issues and I'd appreciate if some others can do some monitoring of theirs and see what they find as well (e.g. a problem inherent in all the cars or my PCM got programmed by someone that failed math at an earlier stage than I did).

For reference I have a 10/2012 build 2013 Diesel.

Issue 1: Reported vs calculated MPG differ
I reset my total trip information (screen 2) and the odo trip meter at every fill up. After filling up I calculate my MPG average using the "miles / gallons" equation. The PCM/MFD always reports high and it seems to be worse the better my economy is. I've seen from a 4%-13% difference.
I know some of this is a matter of the discrepancy built into the speedo which effects the odo as well, but is that all it is? 13% seems high.

Issue 2: PCM and MFD can't agree
Over the course of a full tank, what my PCM and MFD say the MFG average is differs and the MFD is always the higher value. I've seen it as much as a 1.1 MFG. The miles traveled is always within 0.1 miles (again the MFD is always the higher value) though.

Issue 3: PCM total miles vs ODO trip meter
As mentioned above, I reset both at every fill up. This last time I noticed that ODO trip meter was reporting almost 10 miles more than what the PCM (MFD matched the PCM). This is the first time that really jumped out at me and I have not had a chance to see if it will duplicate itself or not.

Issue 4: Overweighting of single trip data
The other morning going to work I noticed that with 105 miles on the total clock it was reporting a 29.9MPG average. By the time I got to work (16 miles) it was reporting the total average to have dropped to 28.9 (actually went as low as 28.5, but crept back up). Calculating it out (e.g. ((121*28.9)-(105*29.9))/16) that would have required a 22.3MPG average for the 16 mile trip, but the MFD reported a 26.3 MPG average for the trip. I've run the numbers on a few other short trips since that should not have had any significant on the overall values (5-10 mile trips where there are close to 200 miles on the overall calculations) and I continue to see the same pattern (overall changes disproportionally to what the single trip values say).
Interestingly, however, the mileage that would be required to have the impact being shown in the overall numbers is probably closer to the real world numbers if I had the actual data to calculate from (e.g. actual gallons used).

Any input of your experience (especially backed up with some actual values) would be appreciated before I go drop it in my dealers lap (so I know if it's something that I should expect a fix for or if they are all like this).

 

I'd point it out to your service adviser. My MFD and PCM mpg numbers are always identical, right to the tenth. The MFD is really little more than a convenience and safety device. It conveniently mirrors selected data originating from remote modules, to include the PCM. Viewing the PCM while driving requires taking both eyes off the road. Remoting select PCM data to the MFD is a safer way of viewing it. I'll speculate Whatever interface sends PCM data to the MFD would seem to be at fault.

That said, you touched on one main point that skews mpg numbers; speedometer error. I used a GPS app on my iPhone to conclude that I had a consistent 3 mph difference across the board. Speedo says 35, GPS says 32. Speedo says 55, GPS says 52. Speedo says 70, GPS says 67. Given mph is based upon the 60 minute hour, I used 60 mph to calculate my speedo error. A GPS reading of 63 mph correlates to a speedo reading of 60. So (60/63)-1 equals a -4.76% mph speedo error, which translates straight across to mpg error. So when my MFD/PCM reports 36.2 mpg for example, using the correction factor reveals a more accurate 34.5 mpg. Next time I'm in for service, I'm going to ask if they can correct that -4.76% error in the speedometer module.

MFD trip data resets automatically after 3 idle hours. If you stop and restart within 3 hours, the car considers it a continuation of the original trip. If you stop for more than 3 hours, the car considers it a new trip. Make sure you're looking at the correct MFD data by the way. The MFD offers three selectable levels of mpg data, you choose which one with the right thumbwheel on the multi-function steering wheel; (1) mpg since start of this trip, (2) mpg since last reset, (3) mpg enroute to an active navigation destination. On my CD I've never reset (2), as it reflects mpg since new. I'll be using (3) for the first time later this month when I start a 2 week road trip. It's my understanding that the 3 hour reset feature doesn't apply, so I should be able to use (3) to track the vacation mpg over the whole 2 weeks.

//greg//

 

Thanks. My assumption is that all that information should be getting fed from the same central source, but I've seen some pretty silly system designs in my time so you never know

I'm surprised yours is consistent. Typically it's a fixed variance so you'll see effectively no difference at low speeds and the visible gap widens at higher speeds.

You probably won't have any luck getting them to remove the discrepancy as the MFGs build that in on purpose (varies from about 3%-10% in my experience). Unless it is outside their tolerance they usually won't mess with it.

The reason they build this into the systems is because there are some pretty painfully penalties in some locations around the world should it be found that the speedo is under reporting the speed. So they build in the discrepancy to accommodate reasonable tire/wheel changes (encompasses factory options at a minimum) as well as some fudge factor.

I thought it was 2 hours, but regardless yes I am aware of the different views (I labeled them appropriately in my issue descriptions) and am comparing apples to apples when talking about the discrepancies between the MFD and PCM.

My understanding is that only shows the value for the current planned route. Once you move onto the next route I believe it gets reset. If my understanding is correct, it won't give you the whole vacation value like you are expecting. I could be wrong though as I haven't used it as I've only used the Nav system a couple of times and it has always resulted in me being close to putting my fist through the screen...

 

Somewhere I have a picture of a 997 instrument cluster with more miles showing on the trip meter than on the odometer. Never quite figured that one out.

 

I called my dealer today about getting it in to have this stuff looked at and I think he is trying to blow me off both in that he supposedly can't get me in until the end of the month and in what he was trying to tell me (there is also a bit of recent history that lends to me questioning the level of service here).

I can buy that the speedo/odo might be getting their data directly from the wheels (though I thought those days were gone), but he is also trying to tell me that the MFD and PCM are getting their data feeds from separate sources (he is supposed to call me back after talking to his tech to find out what those sources are) and thus some variance could be normal. It's certainly possible that they could have separate sources, but as programmer and part time hardware hacker it just doesn't seem reasonable for a bunch of reasons.

Does anyone know for sure where the PCM and MFD get their data?

 

I don't know where they get their feeds from but I can confirm (checked it yesterday) that my MFD and PCM matches 100% on the instantaneous data. I also checked the PCM to see if there was an option for data source and could not find one so I think it must be coded in the PIWIS and PIWIS II systems.

Also I asked two different service managers (from local shops I trust) about recalibrating the speedo and odometer to minimize the delta between observed and actual -- mine is ~ 2mph constant high on the CD and ~ 3mph constant high on the 997.2. Both told me it was not adjustable. Sounds like it would require some sort of software update and that there are no plans for such and update.

 

Called another dealer yesterday afternoon. Not only did they not give me a run around about different sources, they had appointment for me this morning and a loaner for me to drop it off last night. A good dealer (which is ironic as these guys have a bad history for how they used to treat customer cars and their sales used to be real slime) really makes a big difference...

Anyway, they saw what I was talking about but don't have an immediate explanation. So they re-flashed the PCM and we'll see if the problem persists. I'm not sure I expect this to fix the issue, but I understand this is the simplest thing to try. At least they are willing to try something...