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 11-03-2003, 07:41 PM #1 rewecow   Posts: n/a Calculating Gas Mileage? I am looking for a formula to calculate a car's instantaneous gas mileage with variables such as friction, weight, air resistance... if someone knows it by heart or could lead me in the right direction to find it, it would be much appreciated.
11-03-2003, 08:24 PM   #2
vwhobo
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Re: Calculating Gas Mileage?

Quote:
 Originally Posted by rewecow I am looking for a formula to calculate a car's instantaneous gas mileage with variables such as friction, weight, air resistance... if someone knows it by heart or could lead me in the right direction to find it, it would be much appreciated.

The simplest way to determine instantaneous fuel milage would be to divide fuel flow (gallons per second) by vehicle speed (feet per second). Now the question is how do you obtain those measurements? Speed is relatively easy but measuring fuel flow is not.

What is the purpose of this exercise, there may be a much easier way to find whatever it is you're trying to answer.
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 11-03-2003, 10:40 PM #3 SuperJew I like pie.     Join Date: Aug 2003 Location: Suburbia Central, New York Posts: 2,584 this is what i just thought of doing....might actually work..... 1) find out how many gallons ure car's tank holds 2) simply drive wherever 3) when you gas up next, reset your trip odometer and drive until you run out of gas 4) when you run out of gas, do the math: miles on odo/ (divided by) gallons in tank. __________________ CF's Resident Pimp Slapper Pimp Slap Count (as of 30 May 2004): 34
11-03-2003, 10:48 PM   #4
vwhobo
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Quote:
 Originally Posted by SuperJew this is what i just thought of doing....might actually work..... 1) find out how many gallons ure car's tank holds 2) simply drive wherever 3) when you gas up next, reset your trip odometer and drive until you run out of gas 4) when you run out of gas, do the math: miles on odo/ (divided by) gallons in tank.

SJ, that's not exactly the best way to check mileage. Especially if you don't like to run out of gas.

1. Fill tank completely full

2. Make note of current odometer reading

3. Drive most of the tank out

4. Refill tank to same point

5. Again make note of mileage on odometer

7. Divide that number by amount of fuel used

8. Presto, fuel milage

This however doesn't answer the question as asked. This person wants instantaneous mileage, as in real time. Not quite so simple.
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 11-03-2003, 11:11 PM #5 rewecow   Posts: n/a I don't have access to the car (Lexus RX 300) and can't find any chart of its gas mileage on the internet. That is why I am looking for some kind of formula where I can get an approximate graph of its gas mileage.
11-03-2003, 11:14 PM   #6
vwhobo
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Quote:
 Originally Posted by rewecow I don't have access to the car (Lexus RX 300) and can't find any chart of its gas mileage on the internet. That is why I am looking for some kind of formula where I can get an approximate graph of its gas mileage.

So what you're saying is you don't need instantaneous mileage? You're just trying to figure out the fuel mileage. Yes, no, maybe, which is it?
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 11-03-2003, 11:38 PM #7 rewecow   Posts: n/a I'm looking for a chart of instantaneous gas mileage vs speed or RPMs... I'm thinking now if I can find that chart for any car it would be nice.
11-03-2003, 11:59 PM   #8
vwhobo
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Quote:
 Originally Posted by rewecow I'm looking for a chart of instantaneous gas mileage vs speed or RPMs... I'm thinking now if I can find that chart for any car it would be nice.

Think about what you're asking for for just a minute. A "chart" to give instantaneous fuel mileage. How exactly could all operating parameters for your car (any car) be combined and then charted? How do the developers of this "chart" know what the exact condition of your internal engine parts are, weight of the vehicle, wind conditions, quality of fuel, etc, etc, etc? There are so many different combinations and possibilities it boggles the mind.

This is a question you have given little thought to prior to asking. I'm always happy to help as are many others here but the question has to be answerable. Think about it.
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Thanks for the pic, jedimario.

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 11-04-2003, 12:39 AM #9 rewecow   Posts: n/a I'm not sure if you understand what I am asking (its probably me not explaining it too well). What I was originally looking for was a graph of gas mileage vs speed for the Lexus RX 300 in really any condition because I just want to see the basic shape of the graph. If I had access to the car, this wouldn't be too hard to make because I'm pretty sure the car displays its instantaneous gas mileage in its display. After realizing that this graph wasn't on the internet, I came here to ask if anyone knew of a formula I could use to recreate this graph with some accuracy. Realizing that a formula like that isn't easily accessible either, I am now looking for the gas mileage vs speed of any car (I just need a basic idea of how this looks). I hope that helps you understand what I am looking for, Thanks for the replies.
11-04-2003, 01:08 AM   #10
vwhobo
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Quote:
 Originally Posted by rewecow I'm not sure if you understand what I am asking (its probably me not explaining it too well). What I was originally looking for was a graph of gas mileage vs speed for the Lexus RX 300 in really any condition because I just want to see the basic shape of the graph. If I had access to the car, this wouldn't be too hard to make because I'm pretty sure the car displays its instantaneous gas mileage in its display. After realizing that this graph wasn't on the internet, I came here to ask if anyone knew of a formula I could use to recreate this graph with some accuracy. Realizing that a formula like that isn't easily accessible either, I am now looking for the gas mileage vs speed of any car (I just need a basic idea of how this looks). I hope that helps you understand what I am looking for, Thanks for the replies.

You're still asking for unobtainium because there are too many variables. The closest way to explain it would be to say the higher the speed, the lower the mileage. And even that is a generalization that can be disproven.
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 11-04-2003, 04:04 AM #11 rewecow   Posts: n/a I have seen cars that calculate instanteous mileage... so getting a graph of it would just require you to sit in the car and chart what it says.
11-04-2003, 04:50 AM   #12
vwhobo
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Quote:
 Originally Posted by rewecow I have seen cars that calculate instanteous mileage... so getting a graph of it would just require you to sit in the car and chart what it says.

That's not the dumbest thing I've ever heard anybody say but it's close. So tell me this. If you already have the answer, why are you wasting time asking the question?
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Thanks for the pic, jedimario.

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 11-04-2003, 06:57 PM #13 theman352001 --- Voice of Reason ---   Join Date: Nov 2002 Location: Wisconsin Posts: 893 As vwhobo already said, there are too many variables. Welcome back by the way. I have a vehicle with an instantaneous gas milage monitor and here is an example of how different the readings could be.... Say at 50 mph and holding steady the milage is around 20 MPG. If I am speading up slightly the MPGs will drop, if I am slowing down, the MPGs will go up. At some points I have been getting over 70 MPG while coasting down a hill and my foot not on the throttle. The same thing applies while you are starting from a dead stop. If you start out slower at say 1/4 throttle, your milage will be one number. If you stomp on it from a stop, your milage will be much less. Now you could get theoretical, which it sounds like you want to do, and calculate the vehicle's coefficient of drag, rolling resistance, fuel efficiency and make some assumptions about wind and acceleration and come up with some sort of graph that would show you fuel consumption needed to overcome frictional forces inherent in vehicle travel. This covers about 6 pages in my Bosch Automotive Handbook (3rd edition) You can get your own copy at: http://www.bentleypublishers.com/pro...005&subject=24 among other sources. Just do a search on it. Hope this helps out a little. Good luck.
 11-05-2003, 12:30 AM #14 rewecow   Posts: n/a vwhobo I don't know if you refuse to read what I am writing or if you have a serious problem understanding simple concepts, but the fact that I made a statement saying its easy for someone who OWNS a CAR with this instantaneous gas mileage meter to make this chart does not mean that I HAVE the CHART or that I OWN the CAR. I have, very politely, tried to explain this to you about two times now because I understand that you are trying to help me out... but each time you come back with, probably what you consider a witty response, that shows you took no time to even re-read my statement and understand what I am saying. theman352001 I appreciate the link.
11-05-2003, 12:37 AM   #15
vwhobo
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Quote:
 Originally Posted by rewecow vwhobo I don't know if you refuse to read what I am writing or if you have a serious problem understanding simple concepts, but the fact that I made a statement saying its easy for someone who OWNS a CAR with this instantaneous gas mileage meter to make this chart does not mean that I HAVE the CHART or that I OWN the CAR. I have, very politely, tried to explain this to you about two times now because I understand that you are trying to help me out... but each time you come back with, probably what you consider a witty response, that shows you took no time to even re-read my statement and understand what I am saying. theman352001 I appreciate the link.

No, there is nothing "witty" about it. Try more like frustrated trying to explain to someone that what they're looking for is simply impossible to do. Theman also told you as much, only in a different way which I simply hadn't thought of because the concept is so simple as to why what you're asking for is impossible.

As for the Bosch book, it's an incredibly good and informative book if a little hard to read because of the small type. But if you don't understand the simple answers I gave you, you'll never understand what it has to say.
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"Everybody believes in something and everybody, by virtue of the fact that they believe in something, use that something to support their own existence."
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