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Transmissions Compared: Which Is Best To Maximize MPG?

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The new Miata has 160 hp, wider and longer dimensions and a six-speed manual gearbox.

The new Miata has 160 hp, wider and longer dimensions and a six-speed manual gearbox.

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In the old days, choosing a transmission was pretty simple.

With only manual and automatic options, choosing was simply a matter of deciding whether you wanted to stir your own gears, or slush along with zero effort, with the torque-converter option.

Now, alongside the manual option, you can choose cars with all manner of automatic transmissions, from the traditional torque-converter to high-tech dual-clutch units. But which is best for the green-minded driver? We've taken a look at the pros and cons of each.

Manual transmission

Manual gearboxes are actually enjoying a small resurgence in popularity, thanks to consumers wishing to save a little money on purchase price. Luckily, those consumers are discovering that manuals are far easier to drive than they used to be, with light actions, little effort needed on the clutch, and more ratios allowing for better acceleration and better low-revs cruising.

That low-revs cruising means they tend to be pretty economical, particularly in highway driving where some autos hunt around the rev range a little more. And the skilled driver can really make the most of having full gearbox control when it comes to efficiency--manuals are often the gearbox of choice for dedicated hypermilers.

It's not all good news, though. Autos have come along in such leaps and bounds that many are now more economical, and of course even the best manual isn't as easy to use as an automatic. If you're buying used, there's no guarantee that an oafish previous owner hasn't dramatically shortened the life of the transmission by riding the clutch or slamming through the gears, either, so maintenance may be more of an issue.

Torque-converter auto

The traditional auto transmission has plenty of benefits. It's generally pretty smooth, for one--not for no reason are these known as "slushboxes". The latest autos aren't just smooth, but many now offer better gas mileage than their manual counterparts, particularly when offered with more gears--seven, eight and even nine-speed gearboxes are becoming more common.

Mazda SkyActiv-Drive next-generation 6-sp automatic

Mazda SkyActiv-Drive next-generation 6-sp automatic

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On the flip side, you'll often have to pay extra for an automatic--up to $1,000 in some cases, which really bumps up the price of cheaper cars. Also, some lower-tech transmissions on the market are significantly less economical than their manual counterparts. And some would argue that they're still not as easy or cheap to maintain as a manual transmission.

Single-clutch automated manual (or "semi-automatic")

In the past these transmissions were often operated manually with a regular shift lever, but no clutch pedal. More recently, cars like the Smart ForTwo still operate in a similar way, but use an automatic-style lever or steering wheel paddles to change gears.

Theoretically, it has the same benefits as dual-clutch transmissions, but is more mechanically simple. That means economy no worse than a manual, but the ease of driving of an auto.

In practice, they can be jerky to use, with long pauses between gearshifts while electronics and hydraulics operate the clutch and gear selectors. Smoother dual-clutch units have made single-clutch versions nearly obsolete for passenger vehicles, at least in the U.S.


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Comments (10)
  1. I'll take the single gear reduction box for an electric motor:)
    Cheers
     
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  2. I would separate Toyota and Ford hybrid transmission because it multiplies torque with electricity rather than belt and cones mechanically.

    eCVT is a single speed transmission with torque multiplication done with electricity.
     
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  3. Do you in fact know that the Ford hybrid uses belts?
    They consulted Toyota and have licensed or obtained patents which would indicate their hybrid transmission is similar to the Toyota HSD.
     
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  4. Ford never consulted Toyota on the development of their hybrid system; they did however trade some minor patents with Toyota prior to production to ensure there was no conflict. Ford's Gen I and Gen II systems used a powersplit eCVT transmission from Aisin Warner, the same supplier as Toyota, so the transmissions were very similar. Ford's new Gen III hybrids (C-Max and Fusion Hybrid and Energi models) uses a new and more capable Ford-developed powersplit eCVT. The function is similar -- two internal electric motor generators plus an ICE interconnected through a planetary gearset.
     
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  5. Sorry, I meant Toyota and Ford hybrid transmission should be grouped together into electric CVT.

    All other (Honda, Nissan, etc) are mechanical CVT.
     
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  6. True the gear ratio on the planetary gearset doesn't change, but calling a powerplit a single speed transmission doesn't quite do it justice. For anyone interested, I recommend playing with the following demo to understand the relationship between the two motor/generators and the ICE:

    http://eahart.com/prius/psd/
     
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  7. CVTs are slowly but surely becoming the standard transmission on midsize and smaller vehicles. Obviously with hybrids for efficiency, then Nissan jumped in a few years ago, and now Honda is dropping CVTs in their four cyl accords. This trend will continue as the need to obtain better mpg increases and consumers see how simple they are to run.
     
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  8. I don't quite agree with your assessment. I'm not including eCVT's are quite different than CVT's used for ICE's.

    CVT's do have some benefits, but they also have some issues. They are expensive to manufacture as the pulleys require very precise machining. They are not as efficient as they might seem due to hydraulic pumping losses required to keep tension on the pulleys. Honda claims reduced cost and increased efficiency with their new CVT.

    Not everyone is going to CVT. Ford uses DCT's for Fiesta and Focus autos. Others are adding more gears. I don't think CVT's will be a single solution.
     
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  9. Not including hydraulic or pulley CVTs and thinking more long term. It's almost inviteable...almost all CVTs and hybrids/EVs mid next decade for new small n midsize vehicles.
     
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  10. For HEV's, PHEV's, and EREV's, an eCVT's transmission can represent a good solution. Ford did a complete technology scan before deciding to develop a new eCVT. Honda finally is going to field a full HEV/PHEV in the Accord; their new eCVT has a clutch that allows direct connection of the ICE at higher speeds which increases efficiency (similar to the Volt). I haven't seen any details on what Nissan is doing.

    You also can have a full hybrid with a larger version of an Integrated Starter Generator (Honda calls it IMA) with a clutch that allows the electric motor to drive the vehicle without the ICE turning. That's what Hyundai uses (and this type of system is also used in some larger RWD vehicles).
     
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