How much accurate is the car computer as regards avg consumptions.

USCGTO · Jul 10, 2014

I have been driving the same route last 3 days- which also happen to be the first 3 days with my TDi.

Distance traveled - 46 miles.
Average speed 59-60 mph
City / HWY - 6 miles / 39 miles

First day -----56.8 mpg
Second Day --57.4 mpg
Third Day ----58.8 mpg

I know when I fill up, I'll know if I was anywhere closer or not..

I am expecting it to be not VERY accurate?

Lightflyer1 · Jul 10, 2014

Tank or multiple tank averages are pretty close (+/- 2 or 3 mpg). I would doubt your tank average will be close to those numbers. Short trips have very little meaning.

jettawreck · Jul 10, 2014

Lightflyer1 said:
Tank or multiple tank averages are pretty close (+/- 2 or 3 mpg). I would doubt your tank average will be close to those numbers. Short trips have very little meaning.

And until you have several tank averages, a single tank computation won't mean much either. Get a log book and start keeping track, at least until you really know how much the car's average reading is off the real value.

USCGTO · Jul 10, 2014

jettawreck said:
And until you have several tank averages, a single tank computation won't mean much either. Get a log book and start keeping track, at least until you really know how much the car's average reading is off the real value.

I created (MS- Excel) my own log book.

So I am going to be logging the miles - expenses - fuel ups.

Rico567 · Jul 10, 2014

I know that our Passat's MFI is a few mpg optimistic. It's easy for me to just use Fuelly (see average for the car below). I keep an envelope in the glove box with the pump receipts obtained when I fuel up, write the odometer miles on it, and enter that info into Fuelly. Then I not only have accurate mileage figures, but a paper trail for the fuel I've purchased in case there's ever a bad fuel problem.

hybridkiller · Jul 11, 2014

I've confirmed that my MFD indicated average is 7% optimistic. Soon after I first got my Golf I filled the tank, zeroed #2 memory and started stacking up receipts (and no, I didn't include the initial fill-up receipt). After about 2000 miles I calculated actual consumption vs what #2 memory said. I've done this twice now, both times got the same 7% difference.

USCGTO · Jul 11, 2014

7% optimistic doesn't seem too bad.

I have been averaging 57 mph on way to work - nil traffic - 50 miles.
I get 52 on way back with lot more traffic.

Either ways VW promises 43 HWY and I am sure I am getting more than 50 HWY.

hybridkiller · Jul 11, 2014

I'm not complaining - I just made a 600 mile trip yesterday and MFD said 53.8 avg - minus the 7% I'm still (barely) hitting 50 mpg (w/DSG) so I'm good

USCGTO · Jul 11, 2014

Anyone comment on fuel tank size.

How much is the maximum you've poured in ~ 18.5 gal capacity

compu_85 · Jul 11, 2014

Our Passat reads 1-3 mpg high on the MFA.

-J

hybridkiller · Jul 11, 2014

USCGTO said:
How much is the maximum you've poured in ~ 18.5 gal capacity

I'll never check this with my car (no, not a Passat) because I never let it get below 1/4 tank. I don't know if this applies to TDI lift pumps, but I know in the past it was never a good idea with many vehicles for their in-tank pumps not to be submerged in fuel. I personally can't think of a good reason to risk any possible ill effects - 500+ miles between fill-ups is plenty for me.

USCGTO · Jul 11, 2014

I guess all those people logging 700 - 800 miles on each tank are making it go extremely low..

jettawreck · Jul 11, 2014

hybridkiller said:
I'll never check this with my car (no, not a Passat) because I never let it get below 1/4 tank. I don't know if this applies to TDI lift pumps, but I know in the past it was never a good idea with many vehicles for their in-tank pumps not to be submerged in fuel. I personally can't think of a good reason to risk any possible ill effects - 500+ miles between fill-ups is plenty for me.

I run most all my vehicles down to empty often, including those with in-tank fuel pumps. Never had one fail yet-ever. Even the '99 Suburban, which are notorious for failing fuel pumps is still original.
My MkIV Jettas don't have in tank lift pumps, so no worry either way.
700 miles and the fuel light hasn't thought about coming on yet.

Lightflyer1 · Jul 11, 2014

USCGTO said:
I guess all those people logging 700 - 800 miles on each tank are making it go extremely low..

Not really, all you need to do is average 45 mpg or better in the Passat and still have several gallons left. I routinely put in 17 or 18 gallons each fill up. But everyone needs to decide this for themselves. I never had any issues doing the same thing with any of my other cars either including a 2001 Jetta TDI, 2005 Beetle TDI, 2006 Jetta TDI, 1999 Tahoe, or the current 2013 Passat TDI.

USCGTO · Jul 25, 2014

So bringing this back.

I finally found out how to read the car computer for the trip average.

I also managed to reset it when i filled up last week. Currently it shows 53.1 mpg with another 320 miles of range.

I am curious as to how much it'll actually be when I fill up.

USCGTO · Jul 27, 2014

So 52.5 mpg as per the MFI.

And, 48.6 mpg as per the calc at pump.

Optimistic by 8%.

USCGTO · Jul 27, 2014

And, this time I had it at 632 miles..and only got 13 gallons in. . Next time I am gonna fill up at 750 miles.

Lightflyer1 · Jul 28, 2014

The fuel display on the car can also be re-calibrated once you know how far off it is using VCDS. Although IIRC different driving styles can still cause it to be inaccurate (All highway vs all city tanks). It is meant to be a guide and not dead on accurate although it can be very close.

kjclow · Jul 28, 2014

On my JSW, the mfd average #2 is usually a little lower than hand calcualtions, 1-2 mpg. On the Golf, it's the other way around with a slightly higher reading of 2-3 mpg. I have about half the miles on the Golf compared to the JSW, so that might also have an effect, but won't really know that answer for another three years.

foggedz · Aug 8, 2014

In my 2013 Jetta TDI the computer MPG is consistantly off just over 1%. Usually the computer will say I got 46-46.5 MPG, and actual numbers entered in to fuelly say I got 44-44.5 MPG.

John Kuhn · Aug 8, 2014

MFI error

On my 2014 JSW, for the first four tanks, the error has been pretty consistently 2.6 mpg (reading is higher than actual). Actual mpg 48-51 on those tanks.

hybridkiller · Aug 9, 2014

foggedz said:
In my 2013 Jetta TDI the computer MPG is consistantly off just over 1%. Usually the computer will say I got 46-46.5 MPG, and actual numbers entered in to fuelly say I got 44-44.5 MPG.

That's actually more like 4%, fwiw.

Lincoln · Aug 11, 2014

Mine's currently running ~2-3 MPG optimistic, but it's been lower, and even pessimistic in the past.

As for when to refuel, I like to find a station shortly after the light comes on. That leaves plenty of distance before running empty (in case of emergency or lack of nearby stations), without excessive stops for filling up. I normally put in right around 16 gallons and fill up at around 800 miles.

jhinsc · Aug 13, 2014

On just two tanks of fuel, I calculated it's off by about 5%, about the same as Toyota Hybrids. I track each tank on fueleconomy.gov and Fuelly. On the Hybrid forums, some owners were very upset it wasn't more accurate, and a few insisted (engineers) that MFI is the most accurate reading of mpg's, but if that was true, then why do manual calculations ALWAYS show less? You would think it would vary from under to over if MFI were truly accurate.

John Kuhn · Aug 13, 2014

A predictable (in 4 fills) 5% error certainly agrees with what I'm seeing on my 2014 JSW.

USCGTO · Aug 13, 2014

yes..I tested by 4 fuel ups..
8% off and 7% off

Neild5 · Aug 13, 2014

just got back from a 2100 mile trip, heavy on windy county roads and heavy traffic. MFI claimed 43.3 MPG, the calculator came up with 43.29. Looked good to me!

SkeeterMark · Aug 14, 2014

Mine has varied from a low of 0.3% to a high of 9.7%. Though I try to fill the same way each time, there is always going to be some variance there when calculating actual numbers on a single tank. But after many fills, this will all average out, and after 88 fills, the lifetime average discrepancy is 4.28%.

It has NEVER reported a lower mpg than the actual calculated number.

SoTxBill · Aug 14, 2014

most all of mine have been 2 mpg optimistic..

Whiteboy84 · Aug 18, 2014

09 Jetta TDI. My readout is always about 2 - 3 mpg higher than my manual calculations.

How much accurate is the car computer as regards avg consumptions.

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