MFD MPG not really accurate??!!

[statsanalyst]

After three weeks of ownership I had the opportunity to refill with a resulting 48 mpg on the MFD. Hand-calc shows ~43 and change. Since the initial tank was a dealer fill, I doubt that they crammed as much fuel in as possible.

Anyone else notice this significant of a spread between MFD and hand-calc?

Also removed the filler restrictor prior to filling and had no issues w/removal or refueling.

A couple of side notes and off-topic:

Unblended fuel is difficult to find in West Texas with most stating "may contain up to 20% biodiesel blend."

There are also two unmanned pumps on Loop 430 in Littlefield, TX. One pumps 100% biodiesel and the other is a straight dieselpump . A local company manufactures biodiesel and retails it from that location.

 

[RDC98tdi]

That's about right as far as the percentage off. After filling up for a fourth time, I have found a consistent 8% higher MFD reading than actual miles per gallon at the pump. Others have reported anywhere from 5-10% deviation, but the computer is always optimistic.

 

[kjake]

It can be adjusted with VCDS in 1% increments. I kept track of my fill-ups with Fuelly and then compared them to the MFD readout and after a decent sample size, calculated the percent difference. I believe mine is set to 107% to lower the mpg readout to somewhere closer to my reality.

 

[d2305]

I set mine on temperature as it's worthless.

 

[bubbagumpshrimp]

I set mine on temperature as it's worthless.

Same here.

 

[ruking]

After three weeks of ownership I had the opportunity to refill with a resulting 48 mpg on the MFD. Hand-calc shows ~43 and change. Since the initial tank was a dealer fill, I doubt that they crammed as much fuel in as possible.

Anyone else notice this significant of a spread between MFD and hand-calc?

Also removed the filler restrictor prior to filling and had no issues w/removal or refueling.

A couple of side notes and off-topic:

Unblended fuel is difficult to find in West Texas with most stating "may contain up to 20% biodiesel blend."

There are also two unmanned pumps on Loop 430 in Littlefield, TX. One pumps 100% biodiesel and the other is a straight dieselpump . A local company manufactures biodiesel and retails it from that location.

I do not see an incongruity. What the 48 mpg (average mpg I assume?) shows is that YOU are driving @ a 48 mpg average consumption rate.

..."I doubt that they crammed as much fuel in as possible."...

How much fuel is "crammed" is really immaterial to "your" rate of consumption. Now if you are trying to get the (pen and ink) average mpg over X miles, i.e., tank fulls, etc. then YES you should start with known values. In that sense, it does not matter where you fill to, just as long as you do it consistently. THEN you can start to compare it against the computer. So for example, I fill until first click off. Some folks like to fill to a point certain they can see, i.e., right around the fuel neck. To me, that is too much effort and truly you take a chance in over riding the fuel system's safeguards. Another, not all dispensers put out PRECISELY one gal. Also most stations have in the fine print that fuel volume is not guaranteed as temperature can and does vary: all the time. An easy example , what happens if one fuels (to the gills @ oh dark thirty) @ 35 degrees and the TX desert sun gets the macadam to 135 degrees? Another is altitude, etc.

If the TDI is new I would only use the mpg as an FYI mpg normally gets better as it breaks in.

One metric that I think might be of interest is to establish app how much the tank takes pretty close to right after the low fuel lamp/buzzer goes off for YOUR vehicle. One of mine is normally @ 12.1 gal/14.5 gal oem stated capacity, another is @ app 23 gal/26.4 gal, then refill ASAP.

 

[Joe_Meehan]

Remember that the speed and miles recorded are likely to be inaccurate. Car manufacturers tend to set the car up so it will indicate a higher MPH than actual. That way they will not have to fight with you if the reading happens to be off a little on your particular car and you get a speeding ticket when you thought it was legal.

 

[pdt165]

Remember that the speed and miles recorded are likely to be inaccurate. Car manufacturers tend to set the car up so it will indicate a higher MPH than actual. That way they will not have to fight with you if the reading happens to be off a little on your particular car and you get a speeding ticket when you thought it was legal.

Yes, it is true that the speedo is normally indicating higher than acutal. however there have been lawsuits agains the odo. Car companies got into trouble with warantee laws when the mileage wasnt accurate. In my particular car the speed is off by a few mph of my garmin. However on a long (few hundred mile) trip the distance traveled was off by way less than 1%.

 

[GAZNRN]

My JSW computer is consistently 4-5 MPG over the hand calculation of MPG. Funny that the computer on my Mustang is always .5 MPG off at the most

 

[Joe_Meehan]

My JSW computer is consistently 4-5 MPG over the hand calculation of MPG.

That is intentional. The manufacturer does not want to be taken to court if the event that the settings are off a little resulting in a possible dangerous situation, or just increasing the likely hood of getting a ticket.

 

[pdt165]

My JSW computer is consistently 4-5 MPG over the hand calculation of MPG.

That is intentional. The manufacturer does not want to be taken to court if the event that the settings are off a little resulting in a possible dangerous situation, or just increasing the likely hood of getting a ticket.

He said MPG, not MPH.

 

[DieselMann99]

Can I ask a real dumb question: why is the mpg info on the MFD wrong at all? Why does it read high? After all, I'm sure VW engineers can make it accurate if they wanted to.

Is it simply just VW marketing trying to make the car appear to be getting better mpg than it's really getting? I think I know the answer to that.

 

[DieselMann99]

How does the mpg calculator on the MFD work? Is there a fuel flow device installed in one of the fuel lines?

 

[jbright]

I reset the #2 function on my MFD at each fill up. It varies from spot on vs. hand calculated to + or - 1 or 2 mpg. It's not perfect but close enough for me to have a good idea of what mileage I'm getting for any particular tank. Maybe the newer cars use different electronics. I have no complaints after 85,000 miles.

 

[detroitmike]

I have my fuel consumption adjusted w/ VCDS to +3% I believe. I measured the variance of MFD vs. Hand Calc over 10 fuel ups.

Its about as accurate as I can get it. 4% would be too much. Its usually very close but I still get fliers when switching fuel stations.

Comparisons are listed in my Fuelly below.

 

[shepi1]

I'm averaging 41.5 by hand, the Jetta's computer is about 8% higher. As long as I know the difference exists, I'm good with it. I can make the adjustment in my head.

 

[DieselMann99]

I'm averaging 41.5 by hand, the Jetta's computer is about 8% higher. As long as I know the difference exists, I'm good with it. I can make the adjustment in my head.

I understand that it's simple to calculate in your head. What I don't understand is why VW can't make a MFD that calculates it correctly, and if they can, why don't they?

I guess I'm a bit skeptical of there being a marketing/sales motivation when the error is one that makes the car look better that it truly is. We talk about MFD errors around here on this Forum, but I'd bet that at least half of all Jetta owners, and maybe a lot more, believe the MFD numbers are right.

 

[shepi1]

It might be, and it might be something that will be litigated.

 

[DieselMann99]

It might be, and it might be something that will be litigated.

Tough to prove it's deliberate, even if it is.

Also, tough to prove any damages. VW's likely answer: So it was wrong, so what? How did you get hurt by that? Whatever mileage you're getting, you're getting, regardless of what the MFD says.

 

[RDC98tdi]

My +8% reading on the MFD still stands, 5400 miles later, so at least it's pretty consistent. And mine's still not 100% broken it yet, but I am definitely smoking the EPA ratings on highways. If I set cruise at 75 (GPS recorded, not OBD) then I can easily get 49 per computer, or about 45 real MPG. If I set cruise at 70 MPH per GPS, I get about 54 per the MFD, or about 49-50 real MPG.

I figure that because with my 80% highway / 20% driving mix, my MFD typically shows 47 MPG at the end of a tank (for the entire tank). When I calculate per hand, I always get right at or just above 43. EPA for the manual tranny says 42 highway, so with a mix of city driving in there I am still beating EPA rating and I am happy.

When I hit 90 mph, it quickly drops off, but it so much fun to not drop a gear to pass people

 

[kjake]

7% is working well for me
Both Fuelly and my MFD said 40.2 yesterday when I filled up.

 

[LarBear]

Took a little drive down to Cody,WY, south to Lander, then up to Grand Teton NP, up through Grand Teton into Yellowstone, out at West Yellowstone, and up to Bozeman where we refueled. We travelled 505.5 miles according to the trip meter, and put in 13.09 gal of fuel for 38.6 mpg. The MFD showed ~43 mpg for the ~80 miles to Cody, ~45 mpg for the 90 miles to Thermopolis. We left Thermopolis the next morning and drove to the west end of Bozeman showing ~48 mpg. There was a lot of road construction, it is the season, and not wanting to sit idling for who knew how long I shut the engine off each time. When I did the calculation and found that we'd averaged 38.6 mpg, an MFD being 10% optimistic should have given over 40 mpg. I've wracked my feeble mind trying to figure out why, and can only conclude that when I filled up before we left I'd managed to get lots of foam into tank. Refuelling in Bozeman I was very careful to reduce foaming. Then I did what I should have done before we left (relying on the crap low tire pressure monitor which only shows when one tire is outside of the rest) and checked the tire pressures which ranged from ~29.5-30.5 psi. Pumped them all to 37 psi, and will be diligent with the tire gauge in the future. DUH!!!

 

[skigolfmike]

I just bought a 2013 in Ohio and drove it to Virginia where I work. The computer said 51.5, but the data from the odo and pump said 45.9, ~11%. Will be checking it with more fillups to see if it's consistently off. I'll probably invest in a VAG-com down the road just to be able to correct it.

 

[ezshift5]

What a great thread!

FWIW: I don't have a problem with ANY of the previous responses, observation (or questions). Right on!



ez

 

[oswegoPD]

Mine is easily 10%. I complained when I took the car in for the first 10,000 service. I was told everything is normal. I requested an adjustment because I read on the forum that it can be adjusted in 1% increments. Tech said that was not possible.

 

[WutGas?]

I believe this can be adjusted via VCDS. I turned my MPG gauge off though instead.

 

[I800C0LLECT]

I can't say I care much about the deviation but I suppose the variance is due to the way it's collecting data?

My pontiac grand prix was scary accurate....like +/- tenths accurate. I don't know why though.

 

[DieselMann99]

I can't say I care much about the deviation but I suppose the variance is due to the way it's collecting data?

Call me a cynic if you want, but I gotta believe that VW is capable of making an MFD that displays an accurate mpg number, but that it's better for marketing to have it display a high number. Funny, isn't it, that the error in the number displayed is higher than actual, not lower.

 

[pdt165]

Call me a cynic if you want, but I gotta believe that VW is capable of making an MFD that displays an accurate mpg number, but that it's better for marketing to have it display a high number. Funny, isn't it, that the error in the number displayed is higher than actual, not lower.

I dont think it has anything to do with marketing. All marketing numbers come from EPA esitmates, not "users MPG displays show XXX mpg" Acutally VW would have their best numbers by showing actuall fuelly numbers. The TDI performs better than EPA by a fair amount.

 

[DieselMann99]

I dont think it has anything to do with marketing. All marketing numbers come from EPA esitmates, not "users MPG displays show XXX mpg"

I meant "marketing" from the standpoint of customer satisfaction, not new sales. You and me and everyone else on this Forum knows that the mpg display is false and misleading, but the vast majority of TDI owners aren't on this Forum and are in blissful ignorance about the inaccuracy of the mpg display, and are telling all their friends how the car gets 50 mpg, when it's really only getting 45 or 46. Mind you, 45 or 46 is still good, but 50 sounds better.