How accurate is Mileage on the MFI?

[50pascals]

For the next several months I will be filling my car at least half the time out of a diesel bulk tank with no flow meter / gallon indicator.

Then after reading about how you can tweak the mileage display with VCDS I got wondering, what is my actual mileage vs. displayed?

I drive 47 miles each way to work and display generally says I get 36 going there and 46 coming home. - Hills and prevailing wind would account for the difference.

 

[VeeDubTDI]

The MPG display is generally 3% - 5% high in my experience.

Make sure your bulk tank is clean high quality fuel and not some rotgut farm diesel. These engines are very sensitive to fuel quality - mistakes could cost in the thousands to repair.

 

[50pascals]

Fuel was bought August 31st and has been winterized and treated with borate based stabilizer / preservative.

It's filtered again before going into my tank.

 

[psd1]

Mine has read low compared to actual fills. First tank shows 41.1 actual was 44 MPG, second was 43.3 and actual was 44.7 MPG...your results will vary!

 

[VeeDubTDI]

Mine has read low compared to actual fills. First tank shows 41.1 actual was 44 MPG, second was 43.3 and actual was 44.7 MPG...your results will vary!

Really? I think this is the first I've heard about a Passat without an optimistic MFI.

Your results may vary, indeed.

 

[Mr. Furious]

Really? I think this is the first I've heard about a Passat without an optimistic MFI.

Your results may vary, indeed.

I had my first-ever fillup without an optimistic MFI yesterday. Usually, the MFI is anywhere from .5 to 2 MPG optimistic, but when I filled up yesterday it said 35.2 and the hand calculation was 36.1.

Unique factors for this fill-up:

- I usually fill up at 1/2 tank. This time it was closer to 1/4, so I drove about 50% farther on this tank than I usually do.
- It was mostly commute driving, except for a trip to my mom's house (~60 miles @ 43 MPG per MFI) right before I filled up. Usually, I have at least one day of running-around-in-town driving, but that wasn't the case this time.

 

[TDI2000Zim]

For the next several months I will be filling my car at least half the time out of a diesel bulk tank with no flow meter / gallon indicator.

Then after reading about how you can tweak the mileage display with VCDS I got wondering, what is my actual mileage vs. displayed?

I drive 47 miles each way to work and display generally says I get 36 going there and 46 coming home. - Hills and prevailing wind would account for the difference.

The difference between 36 mpg going to work commute and 46 mpg during your returning home commute may be due to the engine being cold in your commute to work in the morning; so you spend 1/4 of a gallon of fuel merely heating the engine at the start of the day.

That is normal.

 

[psd1]

Really? I think this is the first I've heard about a Passat without an optimistic MFI.

Your results may vary, indeed.

Only the second fill, first was with under 500 miles, second was over 700 miles.

The difference between 36 mpg going to work commute and 46 mpg during your returning home commute may be due to the engine being cold in your commute to work in the morning; so you spend 1/4 of a gallon of fuel merely heating the engine at the start of the day.

That is normal.

Guess that doesn't apply to me. Car is colder at start but I have 1500 feet of elevation loss over 21 miles on the way to work...

 

[TDI2000Zim]

I live in the flat lands of southern NJ, so I speak of my thermal experience.

Congratulations on your coasting to the job

!!!

 

[VeeDubTDI]

Something as simple as a couple of extra stop lights (or their timing) can make a big impact on a short commute's fuel economy. I've noticed this on my way home from work, where I tend to hit about 3 fewer red lights than my way to work.

 

[psd1]

The difference between 36 mpg going to work commute and 46 mpg during your returning home commute may be due to the engine being cold in your commute to work in the morning; so you spend 1/4 of a gallon of fuel merely heating the engine at the start of the day.

That is normal.

This may be the case if the ambient air temp rises significantly during the day, which doesnt always happen this time of year. At least around here!

 

[tdiatlast]

Something as simple as a couple of extra stop lights (or their timing) can make a big impact on a short commute's fuel economy. I've noticed this on my way home from work, where I tend to hit about 3 fewer red lights than my way to work.

^^^this. It doesn't take much stop-and-go to make MPG plummet...

 

[Mr. Furious]

The thing I've noticed is that idling murders the MPG on the MFI, even though diesel engines are extremely efficient when idling. I avoid idling whenever possible, but sometimes I have to wait 10-15 minutes in the car line at my kids' school and it drops like a rock.

 

[psd1]

The thing I've noticed is that idling murders the MPG on the MFI, even though diesel engines are extremely efficient when idling. I avoid idling whenever possible, but sometimes I have to wait 10-15 minutes in the car line at my kids' school and it drops like a rock.

Better be careful divulging info on extended idling...the tree police will be by soon!

 

[TDI2000Zim]

In the People's Republik of Nue Joirsy they can give you a ticket for idiling your diesel engine.

Happily, most law enforcement officers around this utopia can't discern the diesel engine running under the TDi hood.

 

[VeeDubTDI]

The thing I've noticed is that idling murders the MPG on the MFI, even though diesel engines are extremely efficient when idling. I avoid idling whenever possible, but sometimes I have to wait 10-15 minutes in the car line at my kids' school and it drops like a rock.

Hot idling with no A/C won't consume much fuel but idling in cold weather does. If you're idling in cold weather, chances are the HVAC is running the aux heater to keep you comfortable and you're sitting somewhere between 45% and 60% alternator load.

 

[TomB]

Hot idling with no A/C won't consume much fuel but idling in cold weather does. If you're idling in cold weather, chances are the HVAC is running the aux heater to keep you comfortable and you're sitting somewhere between 45% and 60% alternator load.

aux heater using the compressor? Really? First I heard of this. Please explain more about this process.

This is non-sense. If I have the A/C off the compressor should NOT run.

 

[VeeDubTDI]

I never said anything about the aux heater using the A/C compressor. I was comparing fuel consumption in the winter (aux heater running to provide heat, consuming a lot of fuel) to fuel consumption in the summer (no a/c running, consuming very little fuel).

The aux heater DOES NOT run the A/C compressor.

 

[Mr. Furious]

In the People's Republik of Nue Joirsy they can give you a ticket for idiling your diesel engine.

Wow, really? I can see a law like that for, say, big rigs and other large (commercial) vehicles, but for cars? Are idling diesel cars really that much worse than idling gassers?

 

[tdiatlast]

Several states (do your own search) forbid the idling of ANY ICE w/o the driver present.

 

[Mr. Furious]

Several states (do your own search) forbid the idling of ANY ICE w/o the driver present.

Oh, I can believe that, it just seemed odd to prohibit idling diesel cars but not gassers, which is what I thought Zim was saying.

 

[50pascals]

If you have a 1980's era diesel car belching black smoke and sickening fumes then you have something to worry about. Most people who have seen my car don't even know it's a diesel. They will then ask "why doesn't it smell?"

 

[smoav8r]

At the risk of dragging this thread back to the OP's question....
I have found that my MFI is very accurate. Over the last several fill ups it has been within .5 MPG of the hand calculated MPG. I usually top the tank off and don't fill back up until it is below 1/4 tank. 2 MPG is the most it has ever been off vs hand calculations. To me this equates to little more than a rounding error or a variation in how full I actually got the tank.

 

[smoav8r]

I drive 47 miles each way to work and display generally says I get 36 going there and 46 coming home. - Hills and prevailing wind would account for the difference.

It's also warmer in the evening which further contributes to better MPG. I always get better mileage on the way home too. Usually it's a tailwind and the traffic flows better as well. All contributing factors and valid reasons why your MFI is probably accurate. Don't "tweak" something that isn't broke.

 

[50pascals]

I think for me on my drive in I go up several steep hills, that then gradually level off - no real coasting.

Coming home I can coast at least 2 or 3 times.

Prevailing wind is at my back on the way home.

Work is 400ft higher than home, but that wouldn't seem to make a difference.

And the last few weeks it was only marginally warmer on the drive home - if at all!