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
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.
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?
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.
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.
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.
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.
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.
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.
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
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.
I'll second this Rob. It's true because engine friction is constant for any
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.
heavier load, the mechanical (and often thermal) efficiency goes up - hence
mileage. Just don't go all the way to "power enrichment", though. Then you'll
totally blow it.
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.
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