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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. |
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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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? Quote:
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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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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). 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. |
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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. |
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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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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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:
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