slow and thirsty

[BMW3R]

Originally Posted by JeffC

Drive hard and dont look back

This is true.

I am over 12,000 miles and drive my car aggressively. I average 24/20/17 MPG. I was wondering if altitude had anything to do with gas mileage??? I am at 4500 feet and it seems that drivers from sea level suck down a lot of fuel.

[AMOCHOSTO]

Originally Posted by seriousm3

Sounds like your lugging the engine and using too much throttle at low rpm's.

Rev it.

So lugging it will reduce fuel economy?

[gatorfast]

Originally Posted by AMOCHOSTO

So lugging it will reduce fuel economy?

Yes. The car is using more gas but not acellerating as fast as it would in higher rpm's therefore eating more gas for an extended period of time until the car gets up to the speed you are trying to reach.

[drivendriver]

Originally Posted by BMW3R

I am over 12,000 miles and drive my car aggressively. I average 24/20/17 MPG. I was wondering if altitude had anything to do with gas mileage??? I am at 4500 feet and it seems that drivers from sea level suck down a lot of fuel.

Air density decreases with altitude. The engine's oxygen sensor should reduce fuel intake accordingly. You're getting better mileage, but you're also getting lower power.

[AMOCHOSTO]

Originally Posted by gatorfast

Yes. The car is using more gas but not acellerating as fast as it would in higher rpm's therefore eating more gas for an extended period of time until the car gets up to the speed you are trying to reach.

I have reset my fuel consumption indicator and will test this out.

Thanks.

[lucid]

Originally Posted by gatorfast

Yes. The car is using more gas but not acellerating as fast as it would in higher rpm's therefore eating more gas for an extended period of time until the car gets up to the speed you are trying to reach.

Using more gas as compared to what exactly? The throttle position is not the only parameter that determines how much fuel enters the combustion chamber. For a given constant throttle input, by definition, you'll burn more fuel per unit time at higher rpm than lower rpm. Yes, you accelerate faster, but you burn more fuel per unit time. Plus, pumping work and other losses increase with engine speed.

[Nawaaz]

Originally Posted by saildoc

Ok, the car is three weeks old, 450 miles. It is sucking down gas, 13.7mpg total for the distance, yet I am keeping it under 5500 rpm as rec by the windshield sticker. The car feels really sluggish (coming out of a 335i). Is this a typical experience with a fresh motor that needs to seat in? Am I nuts?

Kinda feeling a little let down....

You need to rev to feel the power of course.....But you're probably still in your break-in, so that will be difficult. After you break it in, rev it BABY!!

btw, the poor gas mileage is expected unfortunately, especially if you're doing a ton of city driving.

[gatorfast]

Originally Posted by lucid

Using more gas as compared to what exactly? The throttle position is not the only parameter that determines how much fuel enters the combustion chamber. For a given constant throttle input, by definition, you'll burn more fuel per unit time at higher rpm than lower rpm. Yes, you accelerate faster, but you burn more fuel per unit time. Plus, pumping work and other losses increase with engine speed.

I understand what you are saying. Lets use this is an example: you are cruising at 35mph and want to increase to 70mph. If you are in top gear at 35 and floor it until you reach 70, the engine will be drinking fuel but not getting much out of it because it it being lugged initially. But if you started in in a lower gear at 35 then increased to 70 the engine would still be using a lot of fuel but more efficiently and for a shorter period of time.

Makes sense to me but im no engineer

[lucid]

Originally Posted by gatorfast

I understand what you are saying. Lets use this is an example: you are cruising at 35mph and want to increase to 70mph. If you are in top gear at 35 and floor it until you reach 70, the engine will be drinking fuel but not getting much out of it because it it being lugged initially. But if you started in in a lower gear at 35 then increased to 70 the engine would still be using a lot of fuel but more efficiently and for a shorter period of time.

Well, not really. Again, let's say you are at 35 and want to accelerate to 70, and that you go WOT at high vs low rpm. The acceleration is primarily a function of the specifics of the torque curve (tq vs rpm), and the gear you are in.

In the low rpm case, let's say that you simply stay in gear and floor it. In the high rpm case, you'll need to downshift and floor it. Let's also assume the engine has a relatively flat torque curve like the S65, so when you downshift, the effect of the increased mechanical advantage is not negated by your torque curve falling off a cliff. In other words, the engine made more power when you downshifted, but opareted at higher rpm.

So, you will indeed accelerate and reach 70 quicker in the high rpm case. Let's say that took 5 secs, and you then coasted at 70 at minimum throttle. Let's also say in the lower rpm acceleration case lasted 10 secs. That doesn't mean you consumed less fuel (in total absolute terms) in the higher acceleration case during the 10 secs. Why? Because of what I said above. At higher rpms, your ECU tells the injectors to inject considerably more fuel per unit time because there are more power strokes per unit time. And then, there are the higher inefficiencies of operating at higher rpm. Most of those inefficiencies do not simply scale up linearly with engine speed. Inefficiencies increase at a higher rate with engine speed. This qualitative comparison does not provide us with a definite answer, but it illustrates my point.

That was just a brief WOT comparison. Another way to look at it is to do a low rpm-WOT vs high rpm-part throttle comparison. That would be more meaningful if you wanted to look at different ways of generating the same power output (and hence acceleration). The low rpm-WOT case should provide better fuel efficiency compared to the high rpm-part throttle case as it requires less pumping work and results in less loss to provide the same power output. But, this is a simplified consideration and does not take emmissions, timing, mixture, and other issues into account.

[fpblue]

All M's are thirsty. The M5s and M6s we drove at M school were getting 5 and 6 mpg. This was under hard driving, but I didn't think the MPGs would be that bad.