MPG gauge and downshifting

[rooroo]

Hi all!
I have always driven manual trans. cars. My X is my first auto ('03/3.0L).
One thing I always do when slowing/stopping the car is to downshift. Not only do I get some engine braking, but I have faster engine response in case I need to perform an evasive maneuver.
I do this fully aware that I am trading off fuel economy for my driving style.

I noticed that when I downshift and induce engine braking the MPG gauge won't respond to the increased RPMs. It just goes to the left like I am coasting.
I assumed that the gauge responded to the accelerator being depressed and not the actual engine RPMs. But I recently read an article that mentioned that BMW engines are designed to keep fuel injection at a minimum unless the accelerator is depressed. (even when downshifting?)(I'm not a gearhead and probably misunderstood the article)

Is the MPG gauge accurate or am I right to think that downshifting the X5 will use up more fuel even though the gauge won't show it?
(same thing happens on my Mini Coopers)

anyone know?
Rui

[Obioban]

staying in gear and off the gas will save you fuel, in a stick car or automatic. You've been saving gas all these years.

[2006BMWx5]

Originally Posted by Obioban

staying in gear and off the gas will save you fuel, in a stick car or automatic. You've been saving gas all these years.

and brake pads too!

I always down shift when slowing or stopping, I just wish it was possible to go back to S from M, with out shifting back to D then back to S. On a Mercedes you can hold the shifter over and it will shift back to D in S or C. You can also hold over the shifter to down shift and it will continuley down shift you when you car hits the correct RPMS as you approach a stop. The BMW not so much...you have to shift down each time. This is not a huge deal but it could be better.

[rooroo]

Originally Posted by Obioban

staying in gear and off the gas will save you fuel, in a stick car or automatic. You've been saving gas all these years.

For real?!
That is awesome!!
So does this mean that the increase in RPMs when downshifting still expends the same amount of gas as the engine in idle?
The high revs are a result of gear ratios slowing the momentum of the vehicle?

Is this common knowledge? or is this a result of technological advances in vehicle engineering in the recent past? (i.e. are cars from 80s different?)
I have been told by dozens of people over the years that downshifting will end up using more gas. I've just always considered it well worth losing a little gas in order to have better control of my cars...

[JCL]

When the engine is in over-run, which means that the rpm is higher than the throttle setting, ie engine braking down a hill, the fuel is shut off. You aren't using any fuel in that case. (so it is less consumption than the engine at idle) The exception to all this is that as soon as you touch the throttle, you will use more fuel, since you will be at a higher rpm than if you just left it in a higher gear. The higher use of fuel that people are referring to is from blipping the throttle, and driving at a higher rpm when you take off, not from the time when your foot is off the throttle. I think on balance, any use of manual shifting will use more fuel than the automatic setting, since the auto shift points are optimized for fuel consumption, but judicious use of the steptronic function is perfectly reasonable.

Yes, this is an advance in the last 20 years, so cars from the '80s are different. It is a function of the electronic fuel injection.

What you need to take into account is that though you aren't using any appreciable amount of extra fuel, and you are saving brake pads, you are wearing the engine and transmission more. If you are matching revs so that you don't shock the transmission on downshift, and not shifting down too many gears, it is a negligible effect, but as my father the master mechanic always told customers, brakes are always cheaper than engines and transmissions.

With my own X5, I always used the steptronic for engine braking down a long hill leading from my home, and on trips in the mountain passes. I kept engine rpm at a reasonable level. I did not downshift coming up to a stop sign or traffic light, I let the transmission take care of that.

As a side note, the fuel consumption gauge is not actually measuring fuel consumption, but rather engine vacuum. It roughly correlates to fuel consumption, but not directly. It is there as an indicator to encourage drivers who want to optimize fuel consumption to use a lighter foot on the throttle.

[rooroo]

Thanks JCL, that explains a lot...