On Fri, 16 Feb 2007 21:33:54 +0000, GSV Three Minds in a Can put finger to keyboard and typed:
That's probably true. Automatics are inherently less efficient than manuals anyway - for a manual, optimum efficiency is obtained at somthing a bit below the point of maximum torque while travelling in top gear. How far "a bit below" depends on things like drag and rolling resistance - these are lower at lower speeds, so there will come a point where the engine's torque curve starts to flatten out or approach the peak but resistance continues to build and hence the losses caused by the latter at increased speeds outweigh the gain from the former. Obviously, once the engine goes over the peak of maximum torque then overall effciency starts to drop off quite severely (as by that time drag and rolling resistance are playing a much more significant part). A more flexible, "torquey" engine will reach maximum efficiency at lower speeds than a "peaky" engine and is less efficient when at the peak, but on the other hand it won't lose efficiency so quickly above the optimum. Modern engine design tends towards the flexible end of things, as it's easier to drive in mixed road conditions - less changing of gears is needed in order to maintain momentum, which is a major asset when driving a car with a manual gearbox in low-speed traffic. In an automatic, on the other hand, the system changes gear for you and therefore driver convenience is less of an issue, so the engine can be tuned to be more peaky - something which compensates to a large degree for the automatic's inherent inefficiency in the gearing and also increases the speed where optimum efficiency is reached.
(There's another corollory to this: a manual will usually give you better fuel efficiency around town (provided you drive sensibly), but it's less convenient to drive; an automatic will give you better efficiency at faster speeds on the open road but that's also where the automatic's convenience is broadly irrelevent. So an automatic is better for your wallet, and better for the environment, in precisely the situation when you don't actually need automatic transmission, and where you do get the convenience benefit you're paying for it in increased fuel costs and pollution).
Mark