My parent's car has an overdrive and sometimes they use it for fun or better fuel economy (4 speed automatic). Does this harm anything? It shifts for you to not harm at redline and when its too low the powertrain but if you shift at lets say regular 2500-3500 rpms is it harming the transmission at all?
I mean Manumatic shifting the + - Scotty calls it "manual mode"
Using the Sport Shifter gate doesn't hurt anything. As Joe said, the computer will keep you from over-revving.
Are you talking about the +/- manual gear selection?
(overdrive is something else )
The only thing it will harm is your fuel economy, and maybe a tiny bit more wear. The computer is smart enough not to let you blow anything up.
correction: This is what I am talking about
https://en.wikipedia.org/wiki/Manumatic
Also another question, isn't it more fuel efficient to change gears manually at lets say 2500 rpm than the transmission automatically shifting at redline (or somewhere around there)?
The computer is programmed with the most efficient shift points. 'D' mode will be the most efficient way to drive. If you're not racing, you shouldn't be going anywhere near redline.
Noted.
This is why car makers are slowly eliminating manual transmissions. Computers shift more efficiently than humans.