Archive for Downsizer For an ethical approach to consumption
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Armchair
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While we are on the subject of driving methods...can someone explain to me why automatics have much lower fuel economy than manuals. I'm getting a guilty conscience about my auto... 
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Penny
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Becuase you have less control over changing gear??? I really don't know, it's just a stab in the dark 
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dougal
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It depends on the engineering design of the auto box.
Most are "torque converters" using a fluid coupling to transmit the power. These inevitably have slippage and power loss.
Newer ones often have a 'direct drive top gear' or 'lock up', so that in top, at least, there is no slippage.
One of the reasons that CVTs (belts and conical pulleys) are a better engineering solution (besides allowing the engine to run pretty constantly at its most efficient speed, regardless of the vehicle's roadspeed) is that there is less slippage.
There are semi-auto boxes too, where something very like traditional gear changing is done under auto control, matching speeds for a precise change. These should have the same (small) losses as a manual box.
Sidenote: The Toyota Hybrid concept (Prius, Dave Cameron's Lexus, etc) relies on a CVT auto gearbox design.
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MarkS
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also, the typical auto has fewer gears than the typical manual so will not be in as efficient a gear much of the time.
there is a trade off between increasing efficiency and drivability with adding extra gears to conventional auto boxes. hence although there are 7 and 8 speed auto boxes around feedback is that they are forever changing gear.
I hold shares in torotrak, a uk company eith an interesting ( and I hope soon to be very profitable) slant on the cvt.
for a conventional auto box once the torque converter locks up things are ok, so economy on motorways is often at least as good as a manual.
automated manuals (like thae dsg) can slightly beat a manual under the correct conditions.
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