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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old August 13th, 2002, 14:08   #1
boyelectric
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Default "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

I keep seeing people referring to maximizing your "mileage" or "MPG" by making sure you fill up your tank 'all the way' using the vent trick.

Clarification: Mileage (MPG) will NOT increase by adding more fuel or less fuel. The measurement is specifically independent of total quantity of fuel (miles PER gallon).

Conversely, "miles per tank" is another way of saying "Mile range." This is a figure that is entirely dependent on how much total fuel you have. Neither of these are MPG!

Further clarification: to better assess MPG, one must figure the miles traveled across several consecutive tanks of fuel... this eliminates the issues of how much or little it was filled up at each fueling stop. The more consecutive fillups in a row, juxtaposed against the total miles travelled, the more accurate your numbers.

-a-
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Old August 13th, 2002, 14:41   #2
BongoBrains
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Fuel Economy: 41.1 min. (1st tank); 63.58 max, (B20)
Default Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Quote:
Originally posted by boyelectric:
...Further clarification: to better assess MPG, one must figure the miles traveled across several consecutive tanks of fuel... this eliminates the issues of how much or little it was filled up at each fueling stop. The more consecutive fillups in a row, juxtaposed against the total miles travelled, the more accurate your numbers.

-a-
<font size="2" face="Verdana, Helvetica, sans-serif">Perhaps... but there ARE those of us who use the same pump, parked in the same spot, and vent each and every tank to the brim. IMHO, that is enough of a "method" to figure accurate mileage with each individual tank.

I prefer to compare my mileage to the conditions I was driving under on a tank-by-tank basis. So far, I've got 53k miles worth of data, all vented tanks. I keep notes about tanks that involve unique circumstances, i.e., high speed trips, cruise set at 60 mph, winterized B20, etc.

Not to knock your post, but I think it's misleading to suggest that one "must" average figures over multiple tanks to achieve an accurate figure.
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Old August 13th, 2002, 14:56   #3
boyelectric
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Default Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

There are so many variables when it comes to each fill-up, including the coefficient of expansion of the fuel based on temperature, (5 gallons in the car @ 75deg, 10 from the pump @ 50deg), the angle of the car and how the fuel is hitting the baffles on the inside of the tank and what air gaps are left, etc. etc.

It is not very good scientific process to base any conclusions on one datum (one fill up). I agree that with careful, consistent practice at the same pump, and with other variable attended to, one can much more precise, but not as close as over multiple, consecutive fillups.
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Old August 13th, 2002, 15:02   #4
AutoDiesel
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Default Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

You are very correct!

Too many people are concerned with individual tank fills. It is fun to track mpg's when you have a vehicle that is capable of very good results like our TDi's but the real figure to shoot for is your lifetime average.
I haven't get very good records lately but I do have the figures for the last three tanks.
1) 430 miles / 10 gallons
2) 339 miles / 8.6 gallons
3) 346 miles / 9 gallons
= 1115 mies / 27.6 gallons = 40.39855 mpg

Not bad for 75 to 80 percent city driving.
And a automatic with .184's, TuningBox, K&N panel, and a 2.0L MAF. The 2.0L MAF has lower our mileage about 2 to 3 mpg's out on the highway but our city mileage seems to be better. We used to get 34 to 36 mostly in the city but I haven't seen less than 37.5 mpg for a long time.
I'm happy so far! [img]images/icons/smile.gif[/img]
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Old August 13th, 2002, 15:08   #5
boyelectric
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Default Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Quote:
Originally posted by AutoDiesel:
... the real figure to shoot for is your lifetime average.
<font size="2" face="Verdana, Helvetica, sans-serif">Yep, or even an average for a cross-country trip, where you have very similar driving habits across several tanks.... let's see, with 4 tanks, I've gone from the Pacific ocean to the Atlantic!
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Old August 13th, 2002, 15:18   #6
RiceEater
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Default Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

A lot of posts proudly declare &lt;60 mpg and they say highway but they accomplished this by traveling &lt;45 mph. There are just a few people who scientifically qualify their data with % city speed and highway speed data. There are other variables such as altitude, relative humility, temperature, etc. Most the numbers posted are just worthless drivel.
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Old August 13th, 2002, 15:26   #7
BongoBrains
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Default Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Damn, hit the wrong button, sorry.

[ August 13, 2002, 14:27: Message edited by: BongoBrains ]
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Old August 13th, 2002, 15:59   #8
BongoBrains
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Default Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Quote:
Originally posted by boyelectric:
There are so many variables when it comes to each fill-up, including the coefficient of expansion of the fuel based on temperature, (5 gallons in the car @ 75deg, 10 from the pump @ 50deg), the angle of the car and how the fuel is hitting the baffles on the inside of the tank and what air gaps are left, etc. etc.

It is not very good scientific process to base any conclusions on one datum (one fill up). I agree that with careful, consistent practice at the same pump, and with other variable attended to, one can much more precise, but not as close as over multiple, consecutive fillups.
<font size="2" face="Verdana, Helvetica, sans-serif">I guess that all depends on the "data" you are trying to collect. If you are after lifetime mpg figures, sure, I can give you those. If you are after "900 mile club" stats, I can give you those too. It's really not that difficult. You pump in a full tank of fuel, drive till the next fillup, and repeat the process. The fuel left in the tank (after 900 miles) is minimal, and any temp difference from that previous fillup is really not very significant. The 'miles covered' figure is possibly 'contaminated' by gauge error/tire wear, but that would be the case using either method. The amount of fuel pumped in is a given constant, unless the particular pump has been recalibrated between fillups.

To me, it's kinda like saying, "the temperature outside averages 65 degrees during August". That does nothing to give me a feel for how hot it is when I go outside today. Right now, with the sun out, it's a damn humid 90 degrees. Tonight, it might dip to 70.

I prefer to have data that reflects a smaller set of indicators. Getting lifetime data is still possible, but that isn't the data I'm after. I truly believe that my methods are consistent enough to supply accurate data. I'm not too concerned with trying to convert others to do the same.

Again, I'm not trying to start a war here, but as most people know, the 'data game' can often be played to support whatever findings a person wishes to get. I want data that shows how many mpg I got on my last tank, and do everything I can within reason to get an accurate number for that. Anything beyond that is "splitting hairs" IMO.
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Old August 13th, 2002, 22:36   #9
VW Racer
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Default Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Quote:
Originally posted by boyelectric:
I keep seeing people referring to maximizing your "mileage" or "MPG" by making sure you fill up your tank 'all the way' using the vent trick.

Clarification: Mileage (MPG) will NOT increase by adding more fuel or less fuel. The measurement is specifically independent of total quantity of fuel (miles PER gallon).

Conversely, "miles per tank" is another way of saying "Mile range." This is a figure that is entirely dependent on how much total fuel you have. Neither of these are MPG!

Further clarification: to better assess MPG, one must figure the miles traveled across several consecutive tanks of fuel... this eliminates the issues of how much or little it was filled up at each fueling stop. The more consecutive fillups in a row, juxtaposed against the total miles travelled, the more accurate your numbers.

-a-
<font size="2" face="Verdana, Helvetica, sans-serif">Not to start a food fight here, boyelectric, but it is just as rational to talk about maximizing your fuel "mileage" when referring to a tank of diesel as it is when specifying MPG. Your implication that "mileage" refers exclusively to "MPG" is too narrow. Don't believe me? Well, it's not just my opinion. Type the word "mileage" in at www.wordsmyth.net and you will get four different definitions of the word. And "miles per unit fuel" is only one of them. Besides, a tank of diesel is as rational a "unit of fuel" as any. We accommodate liters and Imperial and American gallons here with minimal confusion, mixing in miles and kilometers along the way. Surely we can discuss miles per tank without overtaxing the imagination.

"Mile Range"? How about just "range" - the standard term when discussing the distance a vehicle can travel on the fuel it can carry.

Finally, IMO, your contention that one must measure fuel consumption over several tanks of fuel in order "to better assess MPG" is useful only for calculating long-term average MPG. It tells me nothing about my city, freeway or mountain mileage. Any time I accurately refill the tank, I can derive MPG accurate to the least significant digit for my current driving conditions.
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Old August 14th, 2002, 07:37   #10
jck66
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Default Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Quote:
Originally posted by boyelectric:
There are so many variables when it comes to each fill-up, including the coefficient of expansion of the fuel based on temperature, (5 gallons in the car @ 75deg, 10 from the pump @ 50deg)
<font size="2" face="Verdana, Helvetica, sans-serif">The coefficient of thermal expansion of diesel is small (0.00046/degF). This means that 10 gallons of fuel at 50 degF will turn into 10.1 gallons at 75 degF. It's really not enough to worry about.

Otherwise, you are right on the mark.

PS I got the number from here . [img]images/icons/smile.gif[/img]
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Old August 14th, 2002, 08:24   #11
DZLguy
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Default Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

"juxtaposed" Today's word of the day... [img]images/icons/grin.gif[/img]
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Old August 14th, 2002, 10:03   #12
boyelectric
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Default Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Okay, I'll concede the "Mileage" term might have broader implications... that just means that people ought to be certain to what it is that they are referring... expecially the difference between MPG and range.

Here's what I am saying stated another way:
1) I don't inherently trust the data point of one fill-up. I have seen many people report "getting 60MPG" one week, and "45MPG" the next (or some example similar). There are too many variables around one fill up. Now if you are reporting 51.5MPG on one fill-up, 50.3MPG on the next, 52.8MPG on the next, etc, etc., and are careful about your variables, I might tend to think your data is more accurate. I take exception to people who get wildly fluctuating mileage and then attribute all manner of things to explain the fluctuations, when the first and most obvious explanations are around the variables of filling up.

2) Adding more fuel to the tank (ie: vent trick) doesn't change MPG one iota.

BTW: THANK YOU for the coefficient of diesel expansion! I was looking for that a long while ago and could not find it. I even asked my cousin who worked for Detroit Diesel, and he didn't know.

-a-
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Old August 14th, 2002, 14:12   #13
Derrel H Green
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Default Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

[img]images/icons/smile.gif[/img]

Quoting exactly from the aforementioned website directly:

THERMAL EXPANSION
Like all liquids, diesel fuel slightly expands in volume as its temperature increases. The coefficient of thermal expansion measures the rate of the expansion. A typical value of the coefficient of thermal expansion for diesel fuel is 0.00046 per degree Fahrenheit. Using this value, 1.000 gallon of diesel fuel at 20F will expand to 1.037 gal-lons at 100F.

Is that actually 0.00046 X 80 (degrees) = 0.0368, or 3.68 percent? [img]images/icons/confused.gif[/img]

Maybe I'm splitting hairs? Do we ever see anywhere near that type of temperature spread? Not here in mild California. [img]images/icons/tongue.gif[/img]

[img]images/icons/cool.gif[/img]

[img]images/icons/smile.gif[/img]
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Old August 14th, 2002, 16:22   #14
pedaller
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Default Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Originally posted by WVWSP61:

Is that actually 0.00046 X 80 (degrees) = 0.0368, or 3.68 percent? [img]images/icons/confused.gif[/img]

Maybe I'm splitting hairs?


Multiply by 1.00046 for each degree.
1.00046 to the power 80 = 1.0374767...

The fuel tanks underground can be significantly cooler than ambient, but I doubt we'll ever see 80 F difference.
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Old August 14th, 2002, 17:51   #15
BongoBrains
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Default Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

So, (to get this straight), to accurately figure my mileage, I should have:

A temp reading from the underground storage tank.
A temp reading from the fuel that remains in my tank. A precise numeric value for the amount of fuel in my tank prior to fillup. A mathematical formula to determine the coefficient of expansion in relation to the existing fuel vs. the pumped fuel. A consistent angle at which the car has to be positioned in order to negate any chance of air pockets in the fuel tank when filling. Readings for ambient air temp at the time of fillup AND during the period of fuel use. Barometric readings during that same time period. Wind direction figures with GPS tracking coordinates to determine true aerodynamic drag figures. Fully charted MPH readings during the full use of tank. Time spent "at idle" also documented and figured into Fuel burn statistics. Etc. etc. etc... ***AND THEN, only average my figures over multiple tanks...***

Do yourselves a favor, if you want to get an accurate mpg reading, simply vent each tank to the brim, parked at the same pump, in the same spot.

Every other factor here will have an "influence" on your readings REGARDLESS of which data indicators you choose to use. Fuel temp induced expansion is going to effect everyone. The "data" is going to be influenced in EITHER CASE.

There are only a few things a person can do to aquire "accurate data". It begins with the three simple steps I mentioned in post #2. The rest of this debate is quite anal.

To each, his own, but please don't come on here with a *condescending* post that "enlightens" us with inaccurate "facts".

Sorry if I've ruffled feathers, but "diplomacy" didn't seem to have any effect...
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