Enigine speed vs fuel economy?

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I drive a 1998 E39 523SE manual (petrol, 100k miles). Mostly urban driving with occasional motorway and rural outings. My OBC says 24.7 mpg. This seems
low from what I've read elsewhere. I'm wondering if it's my driving style which is causing such low mpg. I'm a cautious driver, not slow, but not aggressive either. I tend to change up gears at about 3000rpm or so.
I hear that engines are more effiicient at higher revs so maybe I shouldn't change up so early? Would this improve my mpg, or is it already as good as it gets?
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Fergal wrote:

economy. Try shifting earlier, and demanding less acceleration. Works for me on 5 series.
Bob
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Bob wrote:

Actually, the rpms driven at are not as big of an influence as how hard the acceleration is when starting. Yes, there is some small amount of reduced efficiency due to higher revs, but not as much as you might imagine.
-Fred W
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I've noticed that the mpg gauge (ie the realtime needle gauge not the OBC) will initially dip when accelerating as expected, as the car picks up speed the gauge will level off without me pressing the pedal any further. I guess I just need to figure out the optimum rate of acceleration and rpm to change gear while keeping that rpm gauge needle as far left as possible!
Is it more economical to accelerate quickly up to the desired speed and then change into the highest possible gear, or accelerate more slowly changing gears at higher rpms?
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No. This is called "jack rabbit starting" and it is known to consume more fuel than a more rational start. Slow and easy is the manner in which to save the most fuel. Another thing is to look at the next traffic light, if it is red, let off the gas. There is no point is mashing the pedals to the next red light, you may as well coast. Also, in your own area, and you know the cycle times of the lights, and the next light is green and has been for a long time, you should anticipate that it will be turning red soon, and begin your coasting in that anticipation. Basically, don't race the lights.
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snip
dont know about yours, but on my E30 the needle is just a damped (electronically?) vacum guage....
keith
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wrote:

While it might appear to be a vacuum guage, it really isn't. If it were a real vacuum guage, then it would not tell you that you are getting in excess of 40 mpg while coasting with the throttle closed, then tell you that you were getting zero mpg while stopped with the throttle closed.
It is actually a guage that knows how much fuel is being called for at any given moment, and the speed of the vehicle at that time, and calculates how far the vehicle will travel under those same conditions for any given period of time. If any of the conditions changes, the recalculation for the new conditions is immediate.
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keith wrote:

No, Keith, actually it's not. This has been discussed as-naseum. Do a little research (google groups) on it and you will find out otherwise.
-Fred W
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Yup, feather-foot is a major contributor (to economy).
DAS
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The OBC is an average reading over the entire time since it was last reset. 24.7 for all of your driving sounds about right. There are really two readings, CONSUM1 & CONSUM2, you get to the readings by repeatedly pressing the CONSUM button, and the readings toggle back and forth from 1 to 2. I use 2 for the long term consumption figure, and 1 for each tank of gas. I confirm the reading by doing the math, distance / gallons = MPG. I get a sustained mpg of just over 25mpg.
I think your choice to shift in the range of 3000 rpm is a good one. I think you are getting about as good of fuel mileage as you can get.

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Fergal wrote:

Shift up as soon as possible, keeping the revs as low as possible. Your most efficient speed is the slowest you can go in top gear. The faster you go, the more fuel you'll use. The greater the acceleration, the more fuel you'll use. That's high school physics talking.
I once had an E30 318i ('85, the old one). I usually got around 28 MPG HWY. But a couple of times I had occasion to drive a steady 50 MPH for a few hours, giving me 40 MPG. The engine barely pulled top gear at that speed.
Finally, a 5 Series is a pretty big, heavy car, which takes a lot of energy to get moving. If you're talking about city or mixed driving with much stop and go, 24.7 MPG seems reasonable.
Matt O.
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Matt O'Toole wrote:

some cases) I believe BMW confirmed this on their "e" motors. An engine is more efficient at larger throttle openings.
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That's what I heard as well. I know it sounds counter-intuitive but this is exactly what I was trying to ascertain in my original post.
I suppose I'll try driving around for a while using the fast accelerate/high revs method for a while and see what difference it makes to the mpg.
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Enjoy it while it lasts. You're going to kick yourself back at the pump!
MW

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My VERY unscientific observations on my car seem to show that if the tach is kept in the range of 2500, give or take about 250, the fuel economy is better than if the tach is kept below that range.
While the car might be able to sustain 30mph in 4th, the mileage is actually better at that speed in 3rd. I am not sure that the speed and gear selection actually works out to the engine speed of 2500, but you should get the point. If there is a need to moderately increase the car's speed, then the lower gear does not seem to cause the fuel consumption meter on the speedo to drop quite so much as the higher gear causes. There is a fine line to follow though, because if the rpms climb above about 3000, then the next higher gear should be selected.
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Jeff Strickland wrote:

Very perceptive, Jeff. My engine design text states that maximum fuel efficiency for a 4-stroke engine occurs at a piston speeds of 1200-1500 ft/min - give or take. Forgot what BMW you have, but for my 328, the stroke is 3.31 inches, so
3.31 x 2 /12 x 2500 = 1379 ft/min 3.31 x 2/12 x 3000 = 1655 ft/min
Actually, I 'think' modern materials and design move this range a bit, but I don't have any data. I just like to give my 328 up to 3500 rpm or so.
Frank
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That is a good point to shift into the next gear, but I think it is a bit fast for crusing on the blvd., on the way to get a gallon of milk.
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Fergal wrote:

This will result in poor economy. Try 3/4 throttle with 2K RPM shifts instead.
JRE
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Rob Munach wrote:

I'll second this Rob. It's true because engine friction is constant for any given rpm - mechanical efficiency is useful work minus friction, so when accelerating 'slowly' the engine load is light, and work to overcome friction is a high percentage of the total power required - i.e. mechanical efficiency is low. With a heavier load, the mechanical (and often thermal) efficiency goes up - hence better mileage. Just don't go all the way to "power enrichment", though. Then you'll totally blow it.
Frank
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Raybender wrote:

I think you'll find the difference in engine friction to be small compared to the difference in pumping losses between an open and partially open throttle.
I don't have engine design figures handy, but if you look it up I'm sure this is what you'll find. There's also engine design software available which lets you plug in various figures to see how they affect everything else.
Matt O.
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