porsche pdk main630 01
porsche pdk main630 01Enlarge Photo
Dual-clutch transmission (DCT)
First developed by Audi, dual-clutch automatics are now used by virtually every carmaker. One clutch operates the odd gearsets, the other the even gears. When the car or driver selects another gear, clutch engagement is switched instantly from one to the other. Drive is maintained, so it's quicker and smoother than a single-clutch unit.
Because drive is never interrupted, they can be almost as economical--sometimes more so--than a traditional manual or automatic. Less energy is wasted in moving from one gear to another, and as cars with DCTs often come with gearshift paddles, the eco-minded driver can select their own gears to improve economy.
DCTs are expensive though--picking the DSG option on a Volkswagen Jetta TDI adds $1,100 to the price, and gas mileage is identical--so you'd have to be choosing DSG for a reason other than saving money on gas.
Continuously-variable transmission (CVT)
As the name suggests, a CVT constantly varies gear ratios. This means the transmission can allow the engine to run at its most efficient speed regardless of the situation--high revs for power, low revs for economy, independent of road speed.
There's never any step in the power and torque being delivered, making these transmissions very smooth most of the time. At the same time, many drivers dislike the way a CVT lets engine revs rise and fall irrespective of road speed, and some carmakers even build in transmission modes with artificial steps to make them more appealing.
In terms of economy, they're potentially better than any of the others, at least in low load situations where engine revs never climb too much. A Nissan Versa 1.6-liter with CVT beats a five-speed manual equivalent by 3 mpg in combined EPA testing, at 33 mpg to 30 mpg.
Honda CVT transmission
Honda CVT transmissionEnlarge Photo
Some hybrids, particularly those from Toyota and Ford, use an "electronic CVT", that uses planetary gearsets instead of belts, to combine power between a gasoline engine and electric motor. These are even more mechanically simple, smoother, and thanks to the electric running, even more economical.
Which is best for economy?
It depends. Ultimately, the car you choose has much more bearing on the amount of gas you use than the transmission you equip it with.
Like for like, you'll usually find a manual to be more efficient than an automatic, similar to a single- or dual-clutch automatic, but not as efficient as a CVT. But some carmakers do offer torque-converter autos that beat their manual equivalent, and DCTs can also use less gas.
The best news is that you have more choice than ever, and that the efficiency of any transmission is improving all the time--but with a manual gearbox, the best way to improve economy is by starting with the "nut behind the wheel"--i.e, work on your own technique!