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Old 07-02-2008, 02:52 PM   #11
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Originally Posted by BEEF View Post
given this, you want to lower your rpm level during city driving. possibly use pulse and glide (in neutral) or even EOCing (engine off coasting) these techniques requre some research to make sure they won't damage your transmission but several people on here are claiming big differences after these techniques are implemented.
I drive an automatic. I checked the owner's manual about flat towing and it is not allowed. No EOC for me. I do P&G. Am wondering if I can improve my mileage numbers for my daily work commute. On long highway drives I get great mileage with P&G ... not the case with city/commute driving.

I have been driving in the city with a very light foot, etc. As I mentioned earlier, I am also curious if I've hit a "limit" where I cannot achieve better mpg.
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Old 07-02-2008, 03:21 PM   #12
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As I said, RPM and throttle position together determine fuel rate. At 2000rpm and 20% throttle it will use the same amount of fuel regardless of gear.

I'm not sure how to do the experiment for constant RPM / different gear. Speed will be different. Aerodynamic drag and rolling resistance will be different. Importantly, I'll have to find a long stretch of flat, consistent road with ZERO traffic so I can go along at different speeds without bothering others or being interfered by them. I may be able to do it in the morning using 4th and 5th again, since they're so close together.

The more I think about it and analyze it, the more I'm interested in what the result would be. I've never tried to consider constant RPM, as I have no use for that -- RPM is always a means to an end for me. I do know a lower gear at a given RPM will be less efficient, but I don't know which way the fuel rate and throttle position will go (though I do know that fuel rate and throttle position will both move in the same direction).
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Old 07-02-2008, 07:47 PM   #13
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Originally Posted by theholycow View Post
At 2000rpm and 20% throttle it will use the same amount of fuel regardless of gear.
Until you measure that, you're making an assumption, no?

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I'm not sure how to do the experiment for constant RPM / different gear. Speed will be different.
That's the point.

If my car burns 1 gal/hour at 2000 rpm, then at 25 mph in 2nd gear my mileage is 25 mpg. If I'm in 4th gear and traveling at 60 mph, then my mileage is 60 mpg.

According to the BSFC charts, this shouldn't be the case because fuel flow rate is a function of engine load.

See where I'm going with this?

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The more I think about it and analyze it, the more I'm interested in what the result would be. I've never tried to consider constant RPM, as I have no use for that -- RPM is always a means to an end for me. I do know a lower gear at a given RPM will be less efficient, but I don't know which way the fuel rate and throttle position will go (though I do know that fuel rate and throttle position will both move in the same direction).
I eagerly await your results.
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Old 07-03-2008, 04:41 AM   #14
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another scenereo is to go with other mods to help the p+g that you are doing. aero mods (I know they don't work as good at lower speeds) work on rolling resistance and possibly regrease your wheel bearings.

I think their is a limit to what city mileage you can get but there are still some things you can do. you do also reach a point where the end doesn't justify the means. you could do a full engine/trans swap on it which will take time and money. only you could decide if that would be worth it.
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Old 07-03-2008, 07:29 AM   #15
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Originally Posted by dosco View Post

If my car burns 1 gal/hour at 2000 rpm, then at 25 mph in 2nd gear my mileage is 25 mpg. If I'm in 4th gear and traveling at 60 mph, then my mileage is 60 mpg.

According to the BSFC charts, this shouldn't be the case because fuel flow rate is a function of engine load.
This does make sense to 2000rpm you are using 1GPH so at so at 60 MPH you will travel a little over twoce as far and will still use that 1GPH and your MPG will be a little over twice. Just what your data shows. Sure there are minor detail differences- throttle position, MAP, etc but at either cruise speed you are requiring the same amount of power and the same amount of fuel.
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Old 07-03-2008, 07:30 AM   #16
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Okay, tried 2000 rpm on level ground in 4th and 5th.
4th: ~5.5
5th: ~7
Therefore, throttle position must have been more closed in 4th than 5th.

I stand by this statement:
"At 2000rpm and 20% throttle it will use the same amount of fuel regardless of gear."
Fuel rate is not related to gear, only to RPM and TPS. Exceptions: any lean/rich condition (should not happen), IAT difference (doesn't apply to this question), atmospheric pressure difference (again, doesn't apply).

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If my car burns 1 gal/hour at 2000 rpm, then at 25 mph in 2nd gear my mileage is 25 mpg. If I'm in 4th gear and traveling at 60 mph, then my mileage is 60 mpg.

According to the BSFC charts, this shouldn't be the case because fuel flow rate is a function of engine load.

See where I'm going with this?
Fuel rate WILL differ, because throttle position will differ due to the different gear. A taller gear increases load, which means larger throttle opening for the same RPM, which increases fuel rate for a given RPM. Load is pretty much tied to throttle position.

I really think you're looking at this question the wrong way, using the wrong tool. Fuel rate is fine for choosing a gear to use at a given speed, but if you want to know which gear is more efficient at a given RPM you need instant MPG, not fuel rate. Why do the math to figure MPG when you could just use a much more common MPG meter? The ScanGauge (or any OBDII tool), while unable to measure fuel rate, can calculate it accurately enough to answer the question.
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Old 07-03-2008, 09:41 AM   #17
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Originally Posted by theholycow View Post
Okay, tried 2000 rpm on level ground in 4th and 5th.
4th: ~5.5
5th: ~7
Therefore, throttle position must have been more closed in 4th than 5th.

I stand by this statement:
"At 2000rpm and 20% throttle it will use the same amount of fuel regardless of gear."
Fuel rate is not related to gear, only to RPM and TPS. Exceptions: any lean/rich condition (should not happen), IAT difference (doesn't apply to this question), atmospheric pressure difference (again, doesn't apply).
Interesting. When I can cobble a few sheckels together, I'll get a HF multimeter...

Quote:
I really think you're looking at this question the wrong way, using the wrong tool. Fuel rate is fine for choosing a gear to use at a given speed, but if you want to know which gear is more efficient at a given RPM you need instant MPG, not fuel rate. Why do the math to figure MPG when you could just use a much more common MPG meter? The ScanGauge (or any OBDII tool), while unable to measure fuel rate, can calculate it accurately enough to answer the question.
I have an automatic. I can't 'pick a gear' realistically speaking. Also, if my car is using the same amount of fuel at the same RPM regardless of gear, then I am at a limit in terms of mpg improvements (with regards to my commute). I cannot attain better mileage than what I'm getting now unless I can further reduce RPM ... which is probably not going to happen as I'm driving like grandad already.

As far as instantaneous mpg, I started thinking about this concept when I was on a business trip last month and rented a car that had an instantaneous MPG readout. I noticed that at the same RPM but different speeds I was getting different MPG readouts. I started to think about it, did some dimensional analysis, and figured out that the vehicle's speed is key in determining MPG.

Further, the reason I asked you for data is that you are closer to measuring the actual fuel flow rate than a SG is.

Thanks for the info.

When I setup my doohickey like yours, I'll post some info.
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Old 07-03-2008, 11:07 AM   #18
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another idea to help (if your environment is hilly) the engine load plays a big part in MPG. I have found that if I see a hill coming, I will give it gas and gain maybe 5MPH before the hill putting me at roughly 5 over the speed limit and once I get to the hill lose 10MPH to the crest of the hill putting me at 5 under the limit. this way I don't have such a big load on the vehicle going up the hill even though I will not get as good of gas mileage before the hill.

the net MPG for the entire run will be better. you can set up a scangauge to tell you engine load. that may help you too. also, don't be afraid to upset some people with slower driving. it is a speed "LIMIT". I still don't like going much under it but if the limit is 55, I will go 50 all day long (that is where I personally get my best mileage).

side note: I still consider myself a young person at 28 so this isn't advice coming from and old fart that is just saying to slow down
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Old 07-03-2008, 11:13 AM   #19
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Quote:
Originally Posted by dosco View Post
if my car is using the same amount of fuel at the same RPM regardless of gear
But it's not. RPM != fuel usage. It's only one part of the calculation.

Quote:
figured out that the vehicle's speed is key in determining MPG.
There was a thread about clearing up that question, actually. Here's the thing: Speed affects distance. Fuel rate affects fuel volume used. So, of course speed (Distance Per Time, MPH) is key in determining instant MPG when the other variable you have is fuel rate (Volume Per Time, GPH). You strike "hour" out from both of those and now you have MPG (given that you remove "hour" by proper mathematical rules). MPH@GPH = MPG.

Quote:
Further, the reason I asked you for data is that you are closer to measuring the actual fuel flow rate than a SG is.
For this purpose, the SG's calculation (or that of any car's built-in DIC) is fine and far less work. Fuel rate is important to know other times -- for example, when you're stopped, you still have GPH but you have no MPH.

Does anyone know if the SG calculate DFCO into its average or not? I know it can't directly detect DFCO, but open loop @ closed throttle should always mean DFCO, right?
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Old 07-03-2008, 11:30 AM   #20
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But it's not. RPM != fuel usage. It's only one part of the calculation.
It is, as you said, if the throttle position is the same.

For most driving, my perception is that the throttle angle is damn close to being the same. That's enough for me, although a HF gage setup like yours will actually provide data versus guesstimating on my part.

Quote:
So, of course speed (Distance Per Time, MPH) is key in determining instant MPG when the other variable you have is fuel rate (Volume Per Time, GPH). You strike "hour" out from both of those and now you have MPG (given that you remove "hour" by proper mathematical rules). MPH@GPH = MPG.
I know. I did the dimensional analysis (the math part you are referring to) and figured it out.


Quote:
For this purpose, the SG's calculation (or that of any car's built-in DIC) is fine and far less work. Fuel rate is important to know other times -- for example, when you're stopped, you still have GPH but you have no MPH.
I agree, and someday I'll probably buy a SG. But for now, it's enough to know that the rate of fuel burn is primarily a function of RPM, regardless of speed or gear. Since I pay very close attention to my tach, this is a good way for me to think with regards to improvements to MPG. Until I get a SG, that is.
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