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Manual vs Automatic Fuel & Performance Efficiency - what's new in 2009?


I know someone who just bought an expensive new manual shift BMW estate car (a mid-size station wagon - I think it’s a 530i) and claims it is substantially more fuel efficient than an automatic. What I notice is that it rattles, kicks and jars my bones just like a bus, and he kills the engine a couple of times per trip. Tom and Ray wrote a great, balanced response about automatics in 1993 (Columns, Archive, 1993, September 09). But I do not see an update and a lot has happened since 1993. It seems to me that automatics make better shifting decisions at low to medium speeds, and they also come with thrifty versus power shifting selectors. And even if say, you save 1-2 mpg using manual, doesn’t replacing the clutch cancel out the savings? Note, this car does shut down its engine while stopped in traffic. When I search on the internet, I find no decent studies and mostly I see people writing opinions straight out of the sixties. Is there a good study on auto transmission performance that takes into account the facts about current technologies?

It all depends on the transmission and the driver. A good drive can do better than a poor driver no matter what kind of transmission. That is why don’t don’t see studies as driver differences trump the mechanical differences.

It sounds to me like your friend is brand new to driving a stick shift. Tell him to come up slowly on the clutch while at the some time pressing down on the accelerator. If you do a topic search on this forum on manuals or stick shifts, you will find a few discussions of the differences. Most posters are of the opinion that because automatic transmissions have increased in quality over the years, choosing a manual over a stick shift and vice versa is a matter of personal preference. I for example, like stick shifts because I think they are more fun to drive. However, if I were a taxi cab driver and had to drive 10 hours a day in stop and go traffic, an automatic would be much more suitable.

It’s not that simple. See the original posting by Tom and Ray. Driver differences are inevitable but following that line of reasoning we would not have city/highway mpg ratings. Tom and Ray gave an excellent answer, but it’s out of date.

Hi, my friend is a long-time driver with good skills and to his credit, he does drive economically (meaning “pokey”). If you have not driven in England, you may not have experienced the extreme degree of narrow roads, congestion and hairpin turns that make up even the simplest of driving trips in towns. Between electronic latches, better shifting decisions, better overdrive ratios and lower cost of maintenance I figure that automatics trump manuals now. And I don’t think a hybrid would make sense with a manual transmission. I thought stick shifts were fun in 1975. But having lived all over the Bay Area and the East Coast (and London) I would never own one again. If I feel the yearning for a stick shift, I rent one for few days and get it out of my system :slight_smile: The thing is - he got the car for his wife’s longer drives between towns on major roads, and that’s where the automatic should get better mileage.

It seems to me after reading Tom and Ray’s answer to the question in 1993 that if automatics haven’t trumped (or at least equalled) manuals in terms of highway mileage by now, they soon will. What I think is a more interesting question is whether if someone is exceptionally good at driving a stick shift they can always beat an automatic in terms of mpg regardless of how good the technology gets.

Even if you have years of driving experience, it takes time to familiarize yourself to the car’s throttle response, engine speed wind down, clutch engagement point, and other factors to drive smoothly. Some people simply accept that it is going to rock back and forth and won’t improve on their driving skill. My aunt, who hated riding in a manual, said she didn’t notice that I was driving one until she saw me messing with the shifter repeatedly. Depending on your technique and where you drive, rust may eat the car away before it needs a new clutch.

As far as fuel efficiency is concerned, long haul trucks and tour buses are now available with automated manuals instead of conventional, slushy automatics. You can’t argue with the efficiency of a solidly connected clutch and a lightweight transmission.

I’d say he’s decent on the clutch. You can’t drive 100 feet in his town without another gear change. You are probably an exceptionally good driver; I am looking for the general case. The description of an automated manual (aka semi-automatic transmission) is probably the technology that is either coming out or will come out in consumer automatic transmissions, especially with the rise of interest in hybrids. By the way, automated manuals use torque converters same as automatics. What they do is shift better and have anti-slip methods, which is what I was talking towards in the original post. The anti-slip electric clutch is old technology at this point.

My original question was: what is new in 2009 in terms of technology and what studies have been done to test and compare performance. Is there a good study on auto transmission performance that takes into account the facts about current technologies?

I would suggest that it also would need to consider differences in drivers, driving conditions, weather, traffic, hills etc.

This is a somewhat simplistic answer to your question, but I can tell you that the new 2010 Subaru Legacy and Outback models with the CVT (automatic) transmission get somewhat better gas mileage than the same models with a manual transmission. This is a small bit of evidence that modern technology can–in some cases at least–produce automatic transmission vehicles with superior gas mileage.

The EPA tested both cars. The city/highway mileage is 16/24 for the automatic and 16/23 for the manual. The test is much more realistic than it used to be. I don’t buy your friend’s hypothesis.

Developments in auto transmissions have been dramatic. Manual transmissions have remained pretty much the same, they are now 6 spd rather than 4 or 5 spd. Manual transmissions are still lighter and simplier than auto’s. This means a manual trans is still lighter, more compact, and less complex than an automatic. In the long run that means a slight gas saving and less repairs for a manual vs an auto.

Yet, auto transmissions have closed the gap on weight dramatically. They are more compact, but perhaps not as robust as a manual. As far as gear ratios, computer controls, and lock up torque converters I see auto’s as virtually equivalent to a manual as far as efficiency and fuel economy.

Studies are hard to do because you’d have to have the exact same car, same motor, same gear rations, etc. to have a valid test. Then even if this test is valid it might not be conclusive given all the different auto transmissions out there, CVT and conventional auto trans.

Bottom line, I don’t think a driver who chooses an auto over a manual trans gives up even 0.5 mpg in fuel economy. In fact, some autos may do better in mpg than the manual in the same model car. It is now simply a matter of preference. If you like shifting get a manual. If you want the convienience of a auto shift you are not giving up any mpg to get the benefit of auto shifting.

While I believe that overall manuals last longer, a properly maintained auto can last easily beyond 200K or even 300K miles. I’m not sure we’ll ever see definative studies, but fortunately manuals and auto transmissions will still be available for drivers to choose.

By the way, automated manuals use torque converters same as automatics. What they do is shift better and have anti-slip methods, which is what I was talking towards in the original post. The anti-slip electric clutch is old technology at this point.

There’s conventional automatics with manual option and there’s automated manual. The latter doesn’t have torque converter. It is simply a conventional manual transmission with electronics shifting gears as if the driver would. It is already used everywhere including F1, highway trucks, Ferraris, and Volkswagens.

As others have stated, it is hard to compare manual to automatics because there are too many factors to consider. An automatic’s shift points are not necessarily optimized for all driving conditions. I was making reference to automated manuals just to point out that the mechanisms of a conventional manuals are more efficient compared to conventional automatics since automated manuals are used in long haul trucks. Efficiency is important in that application.

In theory, CVT is the best approach to efficiency. If someone figures out how to build a positive drive CVT, ie not friction driven, then it’ll be used in anything. For now, they cannot survive in high torque applications. For now, I don’t want to find out how much it cost to change a CVT’s drive belt. It’s not like the belt in a Vespa.

The new Subaru CVT uses a very hefty chain drive, rather than belts.