by Milieu intérieur » Fri Aug 02, 2013 10:49 pm
Generally the idea of an auto is you let it do its thing, there are a few situations an individual may choose to limit the highest gear the transmission can change up to. The worse that can happen I suppose is you exceed the maximum recommended RPM of the engine for some period and put a rod through the block or do a crank bearing or something similarly expensive [blow the engine], but you'd have to be....... in some way not to hear the engine screaming and/or not see the tacho/rev counter redlining. This can even be done in a manual, like you just had an argument with the partner and drove away insanely. But then the driver may want the other to hear the car scream down the road valve bouncing across the intersection without stopping.
Autos use a kickback on the accelerator [pops the auto transmission back a gear} when you need some serious power [well, in the old days] today additionally they probably have advanced pressure sensors and shaft speed monitors and more to make the decisions for you. Maybe even part of or connected into the engine management system. Never had an auto apart for repairs.
Newer cars have very smooth gear changes [and are smarter], but there are some contexts, say like traversing a bit of a hill with a load behind, in which case limiting what highest gear it may automatically change up to is helpful. You can have more throttle response and more power in reserve, plus less delay because it doesn't have to change back a gear, or so many, particularly maybe important of lower powered vehicles, and as fsm stated you can get more engine brake down hills by keeping it down a gear.
In some cases autos can't make best decisions about gear changes, as they can't predict what is coming next or maybe do what is optimal as whatever changes [can't anticipate]. More though you simply get a few unnecessary gear changes that you don't even notice and they don't matter. For example you're powering up a bit of a hill and it's say in second gear and you decelerate some and it changes to a higher gear and labours some. It does a couple of changes that probably aren't required but it doesn't matter.
But there's nothing to stop anyone driving their auto like a manual, just don't pop it into reverse or park while powering along at speed. Does them no good at all. Last case like that I heard of was a constabulary lady taking a drunk elsewhere - had drunk chap in front passenger seat I understand - who decided to do a few gear changes while going down the highway, which may have included reverse gear.