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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