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2012 Fiat 500c Cabriolet. (Michael Bettencourt for The Globe and Mail)
2012 Fiat 500c Cabriolet. (Michael Bettencourt for The Globe and Mail)

Rob's Garage

Do manual transmission cars really save gas? Add to ...

Hello,

I recently rented a Fiat 500, which I really enjoyed. The transmission was automatic, but allows you to shift gears yourself if you want. This might be a nice option for me, because my wife doesn't drive a manual transmission, but I really miss one. Fuel economy ratings are always better for the manual transmission version of a car. If you use the manual-shift feature of an automatic, will you get the same fuel economy as a manual transmission?

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Thanks, Mark

Your question depends on a number of things Mark. Conventional automatic transmissions are typically not very fuel efficient because they use a fluid coupling device that connects the engine to the transmission – called a torque converter (TC), and as the name might imply, it has the ability to change (convert) torque delivery based on the slippage of the fluid inside the TC along with engine speed and load. By their very nature, they are not economical, but they are easy to use.

Mark, you might see a very slight increase in fuel economy by shifting manually when measured against the normal function of an automatic transmission, but measured against a manual transmission you will likely not beat the fuel economy numbers of a stick-shifted manual tranny. As for the first part of the answer, this slight fuel economy increase assumes that the driver shifts the transmission early – but not too early as “lugging” an engine reduces fuel economy also.

All right, that said – to the rescue comes a device called: Dual Clutch Transmissions. Depending on the manufacturer, these racing bred trannies also go by the names:

  • Dual Dry Clutch Transmission (DDCT)
  • Twin Clutch Transmission (TCT)
  • Dual Clutch Transmission (DCT)
  • Dual Clutch Automatic (DCA)
  • DualTronic
  • Direct Shift Gearbox (DSG)
  • Parallel Shift Gearbox (PSG)
  • Porsche Doppelkupplungsgetriebe (PDK)

The cool thing about these transmissions is that they combine the best of both power transmission worlds. That is, they are easy to use because they can shift automatically or they can be shifted fully manually – without the use of a clutch pedal, even though they use a clutch – this is what’s key.

In essence, within one gear case there are two transmissions connected to two separate clutches. The torque output of the engine determines the clutch design and robustness of the build.

Typically, an engine that delivers less than 184 lb. ft. of torque will employ a dry clutch configuration, while engines that deliver more than 184 will use a wet clutch design – the housing uses a special oil for cooling. As the cost of production for these transmissions drops, we are seeing more of them in the market place. What was once the domain of exotics like Porsche can now be found in the Ford Focus – and yes, likely the next version of Fiat’s 500.

Mark, not only are auto-shifting transmissions more efficient, they are very quickly becoming the gold standard by which many auto manufacturers are now using to reach their government mandated fuel economy numbers. There are a number of factors that help DCT's produce these economy numbers:

  • There are no pumping losses caused by trying to rotate the fluid coupling (torque converter)
  • The use of the highly efficient clutch (dry or wet)
  • The use of electronics to perform the shifting.
  • Clean-sheet design now allows engineers to increase the number of gears inside the case. It won’t be long before we see eight and nine speed transmissions.

Average fuel economy increases are in the neighbourhood of six per cent – and that’s compared to a manual transmission. Pretty nice neighbourhood!

According to Laurance Yap, Manager, Public Relations, Porsche Cars Canada, Ltd., “Because the PDK transmission has two clutches, one can disengage as the other one is engaging, so in theory, the shift time is zero, with no interruption to the flow of power. On each clutch, the shift time is approximately 100 milliseconds - so if you quickly clicked down from 5th to 3rd, bypassing 4th, it would take 100 ms.”

There is even an improvement over manual transmission cars with PDK.

For example, on a six-speed manual Porsche 911 Carrera, figures are 11.3 city/7.9 highway/9.8 combined L/100 km, compared with a seven-speed PDK, which gets 11.1/7.3/9.4, Fuel costs for one year are $2,254 versus $2,162 according to Transport Canada. The gap will get smaller when the new Porsche 911 Carrera goes on sale in February. It has a seven-speed manual transmission with similar gear ratios, so there will be almost no difference.

In the past, choosing a Tiptronic transmission, Porsche’s manually shiftable automatic, meant slower acceleration times, while PDK means faster acceleration times. In 2007, a 911 Carrera manual went 0-100 in 5.0 seconds, or 5.4 with Tiptronic, Now it's 4.9 seconds with manual and 4.5 seconds with PDK. Economy used to suffer with Tiptronic (it was only five gears); now it’s better with PDK.

So Mark, if you like the new Fiat 500, but not the transmission, wait for the next iteration and equip it with their proposed dual clutch transmission – the C625-TCT. You will have fun, your wife will love it and the only thing you’ll miss was how boring the drive to work used to be – assuming you can get the keys from your wife.

Send your auto maintenance and repair questions to globedrive@globeandmail.com

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