[egtlover74]

I have put about 80 miles from last night, mostly highway mileage. I have noticed how 5th gear yields better mileage on mpg display at 70 mph. What is the recommended minimum speed for 6th gear? Is it in manual? I haven't had the time read the manual yet. Some cars have recommended minimum speed for certain gears, does e90 have that inforamtion? First time BMW owner so help me out guys.

[PhilS]

Which engine and gearbox do you have?

What is the corresponding RPM for 70mph in 5th and 6th gear?

[onesixeight]

I can't see how you're getting better mileage using 5th vs. 6th gear @ 70mph regardless of what gearbox you have. Are you using the OBC to calculate your MPG or eyeballing the MPG gauge?

The manual makes no mention of minmal speed for specific gears...With these new gearboxes, you can pretty much go from 30 to 150mph in 6th gear.

[egtlover74]

Well, that is why I posted this theread. I drove about 5 miles @ 70 mph in 5th gear and the display stayed steady at 25.8 mpg and then drove about 5 miles @ 70 mph in 6th gear and showed 24.5 mpg. I have manual by the way. Is it possible that 6th gear does not have enough power @70 mph? and that causing less mileage? I am wondering my self.

[johnfkitchen]

At low revs, an engine can be running less efficiently, since it does not purge the previous cycle's waste products from the combustion chambers as well as when the flow is increased a little and this faster flow can drag the next fuel/air charge in more effectively.

Also, the fuel efficiency you measure will be very much affected by how consistent the flatness of the road is, and any head/tail wind.

There are a few variables at work here, but it sounds to me as though you may be onto something, even if it's a small effect.

[LEDZEP]

Originally Posted by egtlover74

Well, that is why I posted this theread. I drove about 5 miles @ 70 mph in 5th gear and the display stayed steady at 25.8 mpg and then drove about 5 miles @ 70 mph in 6th gear and showed 24.5 mpg. I have manual by the way. Is it possible that 6th gear does not have enough power @70 mph? and that causing less mileage? I am wondering my self.

I think what is misleading you here is the digital average MPG on the OBC. This is an aggregate average for all the miles you driven since it was reset. In my Passat, I had a "what you're getting right now" display, but the E90 does not have that (except for the analog guage - which is almost certainly using engine vacuum). You would need to drive several miles in 5th, check the reading and then reset it to 0 and drive in 6th to get a ~~~~ comparison.

[Broken1]

I would drive in 5. Don't shift unnessasarily! I only go into sixth at about 130-140 km/h. My diesel doesn't like to be in 4th at 60km's/hm - learn to feel your car...treat her like a lady

[egtlover74]

Originally Posted by LEDZEP

I think what is misleading you here is the digital average MPG on the OBC. This is an aggregate average for all the miles you driven since it was reset. In my Passat, I had a "what you're getting right now" display, but the E90 does not have that (except for the analog guage - which is almost certainly using engine vacuum). You would need to drive several miles in 5th, check the reading and then reset it to 0 and drive in 6th to get a ~~~~ comparison.

I see, so MPG on the OBC is not what you are averaging now but an average of what you have driven since the reset? Hmm....let me test it this afternoon and I will repost the result. Thanks for the post

[Broken1]

It would probably be even more accurate if you put your cruise control on for the duration of your "test"

[stressdoc]

yes -- use cruise control, reset, repeat exactly the same route, preferably a long one

[ward]

I don't see why you wouldn't be in top gear for highway cruising
why rev your engine higher than you have to. If you're spinning the engine faster you're injecting fuel more often

you'll get better gas mileage in 6th, they gave you that extra gear for a reason so use it!

[dmbigs]

70 in 6th?....try 3rd

[mc68386]

Originally Posted by ward

I don't see why you wouldn't be in top gear for highway cruising
why rev your engine higher than you have to. If you're spinning the engine faster you're injecting fuel more often

you'll get better gas mileage in 6th, they gave you that extra gear for a reason so use it!

There's much more to it than how much fuel you're injecting. Say, hypothetically, you need to get from point A to point B at 25mph, using the least amount of gas. Now, in 1st gear, you'd probably be near or past redline, so it doesn't seem to be your best choice. And in sixth gear, at 25mph, the engine would be seriously lugging, and you'd probably need to floor it just to keep moving...also not the best choice. So, somewhere between 1st and 6th is the best gear for cruising at 25 mph. How can we determine the best gear for a given speed? Or, to put it another way, in what gear is the engine running most efficiently for a given speed? Actually, the answer's pretty obvious - when an engine is running most efficiently, it would be putting out the greatest amount of torque (since less efficiency must result in greater energy waste - and if more energy is wasted, torque can not remain as high.)
So, for a given speed, the most efficient gear is the one that puts your RPMs closest to the torque peak for that particular car.
The E90 330 hits it's torque peak at 2750 RPM, and drops quickly below that level, so if 6th gear puts you below that RPM, 5th gear (or lower) is your best bet for economy.

Unfortunately, my 330 is still a couple of weeks away, so someone else will have to answer the original poster's question... ie, what RPMs do 5th & 6th run at 70 mph?

MC

[e90wraith]

Originally Posted by mc68386

How can we determine the best gear for a given speed? Or, to put it another way, in what gear is the engine running most efficiently for a given speed? Actually, the answer's pretty obvious - when an engine is running most efficiently, it would be putting out the greatest amount of torque (since less efficiency must result in greater energy waste - and if more energy is wasted, torque can not remain as high.)
So, for a given speed, the most efficient gear is the one that puts your RPMs closest to the torque peak for that particular car.
The E90 330 hits it's torque peak at 2750 RPM, and drops quickly below that level, so if 6th gear puts you below that RPM, 5th gear (or lower) is your best bet for economy.

MC

Actually, this is not entirely true. The engine does not always produce the maximum rated torque at a given speed. The published torque curve is the maximum torque the engine can generate at a given speed. To illustrate, let's assume that you are going 35 mph in 4th gear in a manual transmission (so that you are always at constant RPM for given vehicle speed and gear). There are three situations: In situation 1, you are going uphill; situation 2, you are going downhill; and situation 3, you are driving on a flat surface. Does the engine produce the same torque? Absolutely not. The engine produces more torque when going uphill than downhill. In fact, it produces precisely the torque needed to overcome drag (aero, rolling, drivetrain loss, etc.), gravity (could be positive or negative whether the car is going uphill or downhill, respectively), to maintain constant speed (assuming no acceleration). Going downhill, sometimes the engine will be generating NEGATIVE torque if it's used for engine braking. Hence the published torque curve is the MAXIMUM torque the engine can generate at a given speed.

If you were to plot fuel consumption rate of an engine (fuel consumption/time), there are two main variables - 1. rotational rate (RPM) and 2. torque. The torque is NOT the maximum rated torque but the torque load on the engine. So the fuel consumption rate would be a 2-D surface rather than a line.

Now to compute the fuel efficiency of the car, you would need:

miles / gallon = speed (miles/time) / fuel consumption rate (gallons/time)

So the best way to maximize fuel efficiency is to drive as fast as possible with minimum fuel consumption rate - totally obvious.

So how do you maximize speed? Well, you either go to higher gear (hence for each turn of the engine, you travel farther) or speed up the engine (turn the engine faster). How do you minimize fuel consumption? Well, the simple answer is to turn the engine slower. Hence to maximize fuel efficiency at a given speed, you want to be at the highest gear and lowest engine speed. What is the lowest engine speed? It is the engine rotational speed at which it has sufficient torque to maintain constant vehicle speed! (since vehicle speed and torque required to maintain constant speed are inter-related or function of each other). Therefore, as a rule-of-thumb, it is most fuel efficient to be in the highest possible gear for a given speed. The reason why you don't drive in 6th gear at 5 mph is that the engine may not produce enough torque at that engine rotational speed (and is therefore lugging or stalling) AND you want acceptable acceleration (as acceleration in 6th gear at 5 mph will be terrible since it does not generate much MAXIMUM torque at that rotational speed). MC68386 almost had it right. Cars have best fuel economy at the MAXIMUM torque at a GIVEN engine rotational speed to maintain constant speed, NOT the ABSOLUTE MAXIMUM torque on the torque curve.

There are some complications though. As I said before, the fuel consumption rate of an engine varies with BOTH speed and torque. The fuel consumption rate might not be monotonically increasing with both speed and torque. Sometimes, there might be some kinks due to the specific engine design. So the kinks (non-monotonic increase in fuel consumption rate versus engine speed and torque) can cause anomalies such as car being more efficient at a lower gear (say 5th versus 6th at 55 mph). However, this is rather rare, and I suspect that if egtlover74 re-did his experiment with care, 6th gear will indeed be more efficient at 70 mph.

There will be other factors, too, such as if you are looking for the speed at which the car will be most fuel efficient. However, this will lead to a long discussion about shapes of 2-D surfaces and optimization theories, so I'll just stop here.

[ward]

nice......

[e90wraith]

Thanx. Good to know that my six years in graduate school wasn't a total waste.

[egtlover74]

Allright, I finally tested 5th and 6th gear @60,70 and 80 mgh. 70 and 80 mph clearly yields better mileage with 6th gear. However 60 mph still needs to be tested little more. It sees to be lack of power at or below 60 mph when in 6th gear. So I have decided that any speed above 60 is good for 6th gear. MPG display also seems to count the time since the engine start. Bottom line is stick to 6th gear when speed is above 60 mph.

[poiney]

When I'm looking at my "real time" MPG meter, comparing gearing it doesn't always seem to be valid. It also seems that the real time MPG meter is optimistic as compared to my actual MPG as measured the old fashion way. I realize that part of the discrepancy is that at the pump, I have to include more city mileage, but somehow it still seems off.

How much faith do other's place in the real-time MPG meter?

[Jaws]

Originally Posted by poiney

How much faith do other's place in the real-time MPG meter?

It's not too bad. It is good for giving relative information. For example, how much gas is the car using going up a hill in 6th gear vs gearing down, etc.

[Jaws]

Originally Posted by egtlover74

Allright, I finally tested 5th and 6th gear @60,70 and 80 mgh. 70 and 80 mph clearly yields better mileage with 6th gear. However 60 mph still needs to be tested little more. It sees to be lack of power at or below 60 mph when in 6th gear. So I have decided that any speed above 60 is good for 6th gear. MPG display also seems to count the time since the engine start. Bottom line is stick to 6th gear when speed is above 60 mph.

This mirrors my findings. Cruising at anything above 100 km/h I'll use 6th.

[Kenny C]

No one mentioned load. If the car is going on a slight uphill grade the load will be higher. You will use more gas at a constant speed in the same or higher gear if the load is higher. If the load is the same (ie grade of the road, wind, # of passengers in the car) mpg in a higher gear has to be better. At least thats my understanding.

[NFS]

Originally Posted by egtlover74

I have put about 80 miles from last night, mostly highway mileage. I have noticed how 5th gear yields better mileage on mpg display at 70 mph. What is the recommended minimum speed for 6th gear? Is it in manual? I haven't had the time read the manual yet. Some cars have recommended minimum speed for certain gears, does e90 have that inforamtion? First time BMW owner so help me out guys.

MPG will probably be better in 5th if you are going up any sort of incline. I the road is flat it will be better in 6th.

Since no roads in the UK are entirely flat at a steady 70mph, if I had cruise engaged my 330i would probably do better in 5th.