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      02-05-2019, 03:29 PM   #1
JimD
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Gas mileage by transmission type

One of the incomplete stories or lies told about transmission type is that automatics get better mileage. That bugs me because a manual transmission is inherently more efficient in energy transfer than a conventional automatic which has losses in the torque converter (which is why it needs a cooler that a manual doesn't need). But manufacturers seem to disbelieve I know how to downshift when necessary and they gear my manual to turn more rpm in top gear than it would if it was an automatic. The higher rpm overcomes the inherently higher efficiency of the manual and the automatic gets better gas mileage.

DCT is mechanically the same as a manual with no torque converter losses. But it gets geared like an automatic so it would be theoretically equal to the manual and as implemented by car manufacturers better in terms of gas mileage. Mechanically it is always better than a conventional automatic.

So do not accept anybody telling you that an automatic is better for gas mileage. Lower rpm is better but transmission to transmission the manual is better. But unfortunately we cannot easily have them geared the same. You could change the rear end on the manual (or put on bigger diameter tires) but then you loose acceleration. What we need is a taller final drive ratio in the transmission. 5th is so close to 6th in my car I often skip it. We need a wider spread. If I really need to accelerate at highway speeds, I drop to 4th.
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      02-05-2019, 04:04 PM   #2
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      02-05-2019, 04:49 PM   #3
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BMW rates the DCT for our cars to have worse gas mileage than the 6 speed manual. I don't understand why this is the case; especially since the DCT has 7 bloody gears. They could have put in a decent overdrive gear for 7th gear. If I recall correctly, the DCT is rated at 25 MPG highway while the 6 speed manual is rated at 28 MPG highway. My mixed driving confirms this as I would get between 19 to 21 MPG mixed.

After the Dinan tune went in my mixed driving MPG went up to about 23 to some times 24 MPG.
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      02-05-2019, 04:55 PM   #4
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Interesting, it is the gearing but I don't know why. 7th gear is not over drive, it is 1 to 1 and the rear end in a 2013 e88 135i or is is 3.46 to one. With a manual in the same car, 6th gear is overdrive, .85 to one and the final gear ratio is 3.08 to one. So the manual turns a lot lower rpm on the highway with the 135.

I assumed the 135 was like the 128 but it isn't. The combination of the final drive ratio and top gear ratio in the 128 automatic is 2.5 where the manual is 2.73. So the 128 automatic is turning less rpm. Both turn less rpm than the more powerful 135.

So maybe BMW was making value judgments about what each type of car owner would value? 128 owners are more practical and less interested in performance and the most performance oriented are 135 owners with DCTs? Doesn't really make much sense to me.
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Last edited by JimD; 02-05-2019 at 05:07 PM..
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      02-05-2019, 05:08 PM   #5
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DCT here. Do my own shifting and have a lead foot. Get 13 to 15 mpg and don't care. Too much fun.
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      02-05-2019, 06:01 PM   #6
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I think the biggest benefit or argument for a DKG(DSG) or ZF Auto in our cars is that they have 7 or 8 speeds over the manual's 6 speeds/gears. That is how the "autos" are more efficient. Low rpms mean using less fuel.

But for me... I will always drive a manual.
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      02-05-2019, 06:18 PM   #7
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Originally Posted by Dackelone View Post
I think the biggest benefit or argument for a DKG(DSG) or ZF Auto in our cars is that they have 7 or 8 speeds over the manual's 6 speeds/gears. That is how the "autos" are more efficient. Low rpms mean using less fuel.

But for me... I will always drive a manual.
Grew up on manuals and honestly would like one too, but alas, no can do...

PS - gotten pretty used to the DCT though and have grown to love it (as long as I get to drive it in manual mode).
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      02-05-2019, 08:17 PM   #8
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Newer automatics have much less drivetrain loss and more gear ratios, BMW and other manufacturers have eight speeds. Torque converters in all modern automatics have a lock up clutch which eliminates any torque converter slippage when it is engaged

While I am also saddened by the fact that most manufacturers are eliminating manual transmissions from their option lists, the fact is that modern automatics are deffinatly faster and more efficient than a manual.
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      02-05-2019, 09:50 PM   #9
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Also in 6th gear the overall drive ratio on an AT is 'taller' than a MT.
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      02-06-2019, 06:46 AM   #10
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The lockup of the torque converter in modern automatics is a good point I probably should have mentioned. That makes it equivalent in transfer of power to the manual but only when the lock is engaged. It is not engaged while shifting and typically not at low speeds either. The best it can get to is equal, it can never be better in transferring power.

Lots of gears is a bit different point. Every engine has a rpm range that it operates most efficiently. It will not be high in the rpm range due to frictional and windage losses. Having a lot of gears allows the engine to be in that optimum rpm range more of the time. But there is not reason that automatics should have more gears than manuals unless you make the assumption that manual transmission drivers won't shift 8 times to get to top gear. I think Chrysler put a 9 speed into the Cherokee and the reviews indicate the vehicle almost never gets to 9. I drove one as a rental and couldn't tell exactly but it did hunt around at highway speeds trying to find the lowest ration the engine can pull. That sort of messing around would possibly get annoying with a manual.

But my main point stands. Mechanical efficiency of energy transfer goes to the manual. That can be more than offset by unequal gear ratios but there is no need for that to be the case.
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      02-06-2019, 07:06 AM   #11
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I've had a 2011 128i AT and a 2009 128i MT and get nearly the exact same average with a slight tilt to MT
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      02-06-2019, 01:19 PM   #12
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New autos are typically more efficient. That said :

1. Who cares about fuel economy in a sports car?

2. The difference is so small we're talking about spending $20 (annually) more on gas. Worth it.
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      02-06-2019, 03:03 PM   #13
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Any difference in mileage between DCT and manual comes from the additional weight of DCT and the overhead to run the actuators. That's it. The difference when you're rolling down the highway is so small you will literally get better mileage by washing the bugs off your car and waxing it than switching from DCT to manual.
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      02-07-2019, 07:34 AM   #14
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It isn't really the subject of the thread but it seems the 135i gets worse mileage than the 128i regardless of the transmission. That seems logical that a more powerful engine would be worse than a less except that the added power comes by turbocharging. Turbocharging uses energy that would otherwise be wasted in the exhaust to pressurize the engine which increases it's efficiency. That is why GM is putting little 1.3 or 1.4L turbo engines in the smaller cars and Honda went to a turbo 1.5L in the Civic. These smaller turbo motors will get better mileage than the larger engine they replaced. Smaller also helps but the basic addition of turbocharging improves efficiency.

So if was geared the same, and driven the same, the 135i should get better mileage than the 128i, not worse. The torque advantage could also be used to have it turn lower rpm in top gear which would further increase it's gas mileage advantage.
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      02-07-2019, 08:16 AM   #15
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That only makes sense if the turbo engine is smaller. If you have 2 engines of the same basic configuration and displacement, adding turbocharging won’t make it more efficient that the NA engine. Yes, in theory, you are capturing otherwise wasted heat, but that only happens on boost. When cruising, you’re not on boost so you’re just driving a lower-compression engine with higher pumping losses and a lot of additional weight.
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      02-07-2019, 10:31 AM   #16
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This is total nonsense. If both transmissions are driven in reasonable way, the auto will get better mileage. The reason is simple. The auto always completes the shift pattern exactly the same no matter how many times it does it. With the manual, the shift points will vary, because the human operator isn't a computer.
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      02-07-2019, 10:53 AM   #17
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If you google up what Wikipedia says about the otto cycle (the technical term for a normal 4 cycle engine) you will see a reference to improved efficiency with turbo charging. That is just one possible reference, it is an established fact from an engineering point of view that there is an efficiency advantage to turbocharging.

But what you guys are saying has merit too. If you use the extra power of the turbo motor, your mileage will be lower. The efficiency gain is small so the first principal of more power out requires more fuel in still applies. In the case of how it was implemented by BMW in the 135i, any efficiency gain from the turbo was offset by the gearing raising the rpm of the motor. The advantage went totally to performance, not fuel economy.

Weight is also a factor but the weight of the extra pieces for turbo charging is not going to be a big factor. Most of us do not track gas mileage based upon how many passengers we have. There is some small effect but we ignore it. The turbo weighs a lot less than another passenger.

I'm not sure about the point that the boosted engine isn't always on boost is a gas mileage factor. I thought of that too. But the 135i engine is supposed to build boost at just over 1,000 rpm. So there should be some boost nearly all the time. Maybe not at part throttle.

I am confident the key factors to reduced mileage in the 135 are the gearing and use of the added power. If the 128 and 135 were geared the same and driven the same, the mileage would be equal or the 135i would be better. Driving a 135i like a 128i might be pretty difficult, however.
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      02-07-2019, 12:59 PM   #18
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Modern autos should be more efficient than MT, all other things being equal. However the DCT in the 135 has on overdrive and is less efficient on the highway than MT.

Both my DCT 135's get 3 mpg less than my MT 135 used to get with highway driving. All N55 engines.

All comes down to gearing and aerodynamics for highway. Hell my e90 335 xdrive auto, which is a tank, gets 30 mpg on highway.
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      02-07-2019, 01:20 PM   #19
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Quote:
Originally Posted by JimD View Post
If you google up what Wikipedia says about the otto cycle (the technical term for a normal 4 cycle engine) you will see a reference to improved efficiency with turbo charging. That is just one possible reference, it is an established fact from an engineering point of view that there is an efficiency advantage to turbocharging.

But what you guys are saying has merit too. If you use the extra power of the turbo motor, your mileage will be lower. The efficiency gain is small so the first principal of more power out requires more fuel in still applies. In the case of how it was implemented by BMW in the 135i, any efficiency gain from the turbo was offset by the gearing raising the rpm of the motor. The advantage went totally to performance, not fuel economy.

Weight is also a factor but the weight of the extra pieces for turbo charging is not going to be a big factor. Most of us do not track gas mileage based upon how many passengers we have. There is some small effect but we ignore it. The turbo weighs a lot less than another passenger.

I'm not sure about the point that the boosted engine isn't always on boost is a gas mileage factor. I thought of that too. But the 135i engine is supposed to build boost at just over 1,000 rpm. So there should be some boost nearly all the time. Maybe not at part throttle.

I am confident the key factors to reduced mileage in the 135 are the gearing and use of the added power. If the 128 and 135 were geared the same and driven the same, the mileage would be equal or the 135i would be better. Driving a 135i like a 128i might be pretty difficult, however.
It will build boost at 1,000RPM if you want to accelerate, sure. It will also use a lot more fuel than an equivalently-throttled NA engine of the same displacement, because more air is going in (so you can add more fuel). But maintaining speed, you're not building boost because you don't need to or want to. At a constant speed and off boost, there is really no difference. A better example to illustrate your point would be the Mustang, which offers an NA 3.7l V6 and the Ecoboost 2.3l 4-cylinder, making comparable power (300 in the V6 and 310 in the I4). The Ecoboost gets 3mpg better than the V6 across the board while having more peak power and more useable power lower in the rev range. The efficiency gains in the real world come from using a smaller engine to make the same power as a bigger engine.

This entire thread is so theoretical that it's really not relevant to the real world at all. Not saying the discussion isn't interesting, I just don't think you can extrapolate any conclusions drawn here to explain the mileage you see in your car (128i or 135i). But since we're just bullshitting here, I am also pretty sure that the container ships streaming across the Atlantic full of high pressure fuel pumps and water pumps needed to keep the 135i fleet running are negating any theoretical fuel efficiency gains from said 135is!
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      02-07-2019, 03:17 PM   #20
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To me the usefullness is only if you either want to understand just because you want to understand or if you want a solid basis to change things. If you want an automatic transmission then get it. But don't get it thinking it makes your car more efficient. It might if the two alternatives are geared differently but a conventional automatic hurts fuel economy, it doesn't help it (it also means more frequent brake replacement).

The importance of final drive ratio on fuel economy is often misunderstood. I am told that Corvettes turn very low rpm in top gear so that their fuel economy numbers will look better. BMW doesn't do that with our cars. If you don't understand fuel economy and put in a higher ratio rear end, you shouldn't be surprised when fuel economy gets significantly worse. But you might be surprised if you don't understand that relationship.

I'm a mechanical engineer and it bugs me when I read somebody saying something like a conventional automatic contributes to better fuel economy.
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      02-07-2019, 07:57 PM   #21
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Fair enough, Jim. I like understanding things and this is a good discussion.

My father in law has a C6 vette that turns about 1200rpm in 6th at 75mph, where my car is turning close to 3 grand. Both cars accelerate at about the same rate when you put your foot down in 6th at that speed, but the vette gets about 30mpg to my 25. Some of that is aero but a lot of it is rpm like you said. Or maybe all of it is aero, but the aero gain isn’t offset by the fact that it’s a 6+ liter V8 because it’s turning so slowly.
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      02-08-2019, 07:16 AM   #22
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An analogy that helps me is to think of an engine as an air pump. The engine doesn't have to pump the exhaust gas out, the expansion during combustion raises pressure a lot (that is what propels the car) and will move the exhaust out. But the engine has to mechanically draw the air/fuel mixture in. A turbo helps efficiency because to uses otherwise wasted energy in the exhaust flow to do some or all of that pumping. Air moves easiest at lower velocity. Engine rpm raises velocity. So running the engine at higher rpm will reduce efficiency because it increases pumping losses.

The vette example also tells us something about frictional losses versus pumping losses. If frictional losses were more significant, a 300 cubic inch engine would have more difficulty getting to 30 mpg than a 180 cubic inch engine. It does but the rpm difference over-riding the size difference suggests pumping losses are more significant. At least to me. Friction will also tend to increase with rpm, however. But a bigger engine has more square inches of bearing surface and the bearing types are largely if not totally the same so more inches of area should equal more frictional loss.
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