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Old 12-22-2008, 02:24 AM   #28
PedroBMW
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Join Date: Oct 2005
Location: Windsor, Ontario
Posts: 1,206
I'm a powertrain engineer, so I'll try to shed some light from my perspective,

Automatics have worse fuel economy due to the energy wasted while slipping the torque convertor. We try to lock that sucker up as much as possible but often times NVH/driveabilty don't allow it.

Manual transmissions gain fuel economy because there is minimal slip other than when launching from a standstill, typically there are more gear ratios (obviously this has changed with many modern automatics), and the fact that the driver can employ a very aggressive "fuel saving" shift schedule as they desire, they may lug the engine slightly but typically its tolerated by most manual drivers since they are the ones causing it, however you can't get away with that sort of shift schedule on an automatic and still have it be "customer acceptable".

For the resons above are why dual clutch transmissions are becoming popular, they essentialialy manual transmissions shifted automatically using two clutches (one for even gears and one for odd).

SIREX, as far as real data showing the differnence in fuel economy auto vs manual, I have more than you would ever want to look at, we run EPA emmisions cycles daily with emmisions benches that cost millions, trust me its true especially in real world driving like I described above.
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