Fuel Efficiency Calculation (MPG)

[ertzog]

Hurray! a place for econo-freaks. Lets all save the planet a little each day.

So we are all on the same page, come spring and full power diesel supplies, lets all agree on how mileage is calculated.

What I do:
Keep an excel chart with entries for each fillup. MPG is calced by:

(mileage at this fillup - mileage at last fillup)/US gallons added at last fillup

Over the long haul, filling up at different levels of "empty" will even out. But try to keep it a common point on the fuel gauge or your charts will be wacked with anomolies.

I know this has been discussed before but lets air it out for the sake of being able to find it by a search of this new forum.

Is my method the same as everybody elses?

 

[ertzog]

beware! those anomilies will get you every time.

 

[Karl Roenick]

The problem with averaging out is that you don't see what effect your driving patterns have on your mpg.

 

[Ric Woodruff]

ertzog: Don't say anomaly!!! (we have a launch tomorrow).

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Ric Woodruff

Braumeister von Sehr Guter Bier
Since the Last Millenium

 

[JeffT]

I do the same as ertzog, I also use Excel. The one thing that Seatibezatdi has is that danged trip computer. If we had something like that in front of us we'd all have a much better idea as to how to drive economically.

I wish we could come up with a way to at least monitor the flow of diesel remotely via a digital readout. If we could do at least that it would allow us to do basically make the same measurements as Seatibezatdi does.

Is anyone aware of a way for us to accurately measure the flow of diesel from out fuel tank to engine??? There must be devices designed specifically for this application?

 

[Lug_Nut]

Jeff, you (and we) will need two meters. One for the fuel from the tank to the pump, the other to measure the return from the pump back to the tank. The difference is the consumption. I don't know if the pump would last if the return were to be blocked to allow only one meter. The lack of flow through could lead to overheating the pump components.

 

[Dominique Cormann]

I calculate my mileage differently.

I first fill the car to the exact same level each fillup.

Then I record the mileage per tank and the amount of fuel that must be added to the tank to reach the same level I fill to each time.

That way I can fill the car up any time I want, ( I don't have to run the car down to what I think is near empty each time I drive ).

Its much more accurate, and more flexible this way.

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My Project GTD and diesel page
http://kozmik.guelph.on.ca/gtdproject

 

[JeffT]

Johnathan, I understand your point about the fuel line and return line, and I guess I need to understand the system's dynamics more.

How does the return line work?

 

[ertzog]

Dominique: How is that different? I am always filling up to the same point (vented, full). It sounds like you are doing more math in return for the flexibility of filling up anytime you want (yuck, I hate filling up so I put it off as long as possible).

Your method prevents anomilies from short tanks (which I don't have), but how is it more accurate?

Jeff: Differential flow meters get real expensive. They would most likely need to be mag type (non contact), not positive displacement or turbine (would hate to plug the fuel system with your mod) - thats like $2k just for the meter, never mind what you put the signal into.

I'll stick to my charting... What we really need is a way to get/calculate the ECU's fuel consumption value. I doubt that A4's with trip computers have any extra sensors - all the data must come from the ecu......

From what I've heard OBDII does not show much we can use, perhaps the VAG gets deeper into the ecu?

 

[Ric Woodruff]

Gentlemen: All we would have to do is tap into the line going from the fuel filter to the injector pump with a low flow digital flowmeter and tap into the line going back to the fuel filter with a second flow meter.

We wouldn't necessarily even need to get an absolute flow reading. All we would need is to get readings at different speeds, say starting at 40 MPH and going in 5 MPH increments to say, 75 MPH.

Then all we would need to do is divide the flow number (flow into injector pump - flow coming out of injector pump) by the road speed to get a relative flow/speed ratio. The lowest ratio = highest fuel economy.

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Ric Woodruff

Braumeister von Sehr Guter Bier
Since the Last Millenium

[This message has been edited by Ric Woodruff (edited February 03, 2000).]

 

[ertzog]

Uh, yea. That's what a differential flow meter would be.

Not rocket science, just prohibitively expensive.

 

[Autopilot]

ertzog,
I do the same thing with one exception. You stated:

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by ertzog:

What I do:
Keep an excel chart with entries for each fillup. MPG is calced by:

(mileage at this fillup - mileage at last fillup)/US gallons added at last fillup <HR></BLOCKQUOTE>

I take (mileage at this fillup-mileage at last fillup)/US gallons added at THIS fillup.

Since I always fill the same way, that tells me what the MPG was for the last tankfull. If for some reason I do not fill all the way up, I just add the partial fill onto the next full fill and use the range from between the last 2 full fills for the calculation.

I did this for all of the 6869 gallons of gasoline I ran thru my '85 Jetta over 13 years and 225,524 miles. I am kinda anal rententive about recording mileage and fuel consumption. I've even gotten my wife to participate now in her car without complaining.

It was interesting to plot the MPG over the 13 years I operated the '85. The mileage curve formed an almost perfect sine wave, peaking out at 33 MPG every mid summer (July/Aug)and bottoming out at 27 MPG every mid winter (Jan/Feb). I also recorded where I bought the fuel to look for variation that I could assign to fuel sources, as well as unusual situations like long trips with highway only mileage etc. The only thing that made an assignable difference in mileage was seasons and unusually cold spells, like -30F.

I plan on doing the same thing for my TDI hopefully for the next 13 or 15 years.

 

[NBTDImiata]

My 1st post, hope it works right. I agree. I keep the same spreadsheet, note trips and/ or where fuel purchased to try to find patterns or more importantly signs of trouble. My high is 62,which I disregard because it was a low qty fillup on highway miles and likely a short fill. My regarded high is 56, and low 28 on a "track day".
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Autopilot:
ertzog,
I do the same thing with one exception. You stated:

I take (mileage at this fillup-mileage at last fillup)/US gallons added at THIS fillup.

Since I always fill the same way, that tells me what the MPG was for the last tankfull. If for some reason I do not fill all the way up, I just add the partial fill onto the next full fill and use the range from between the last 2 full fills for the calculation.

I did this for all of the 6869 gallons of gasoline I ran thru my '85 Jetta over 13 years and 225,524 miles. I am kinda anal rententive about recording mileage and fuel consumption. I've even gotten my wife to participate now in her car without complaining.

It was interesting to plot the MPG over the 13 years I operated the '85. The mileage curve formed an almost perfect sine wave, peaking out at 33 MPG every mid summer (July/Aug)and bottoming out at 27 MPG every mid winter (Jan/Feb). I also recorded where I bought the fuel to look for variation that I could assign to fuel sources, as well as unusual situations like long trips with highway only mileage etc. The only thing that made an assignable difference in mileage was seasons and unusually cold spells, like -30F.

I plan on doing the same thing for my TDI hopefully for the next 13 or 15 years.<HR></BLOCKQUOTE>



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Daniel
99 NB TDI
90 Miata